An integrated electrochemical sensor–actuator system

An integrated electrochemical sensor–actuator system

Sensors and Actuators A 114 (2004) 65–72 An integrated electrochemical sensor–actuator system Michael K. Andrews a , Murray L. Jansen a,∗ , Geoffrey ...

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Sensors and Actuators A 114 (2004) 65–72

An integrated electrochemical sensor–actuator system Michael K. Andrews a , Murray L. Jansen a,∗ , Geoffrey M. Spinks b , Dezhi Zhou b , Gordon G. Wallace b a b

Industrial Research Ltd., Gracefield Research Centre, Gracefield Road, P.O. Box 31-310, Lower Hutt, New Zealand Intelligent Polymer Research Institute, University of Wollongong, Northfields Avenue, Wollongong, NSW, Australia Received 24 April 2003; received in revised form 24 February 2004; accepted 1 March 2004 Available online 20 April 2004

Abstract The use of bimorph actuators based on conducting polymers (CP) in an integrated oxygen control system is explored with practical focus on the control of a fruit storage atmosphere at 5% oxygen. A low-cost oxygen sensor is integrated with a simple actuator valve to limit the influx of atmospheric oxygen to the storage package. For this task, electrochemical oxygen sensors as used in the study, such as zinc–air cells with output potentials up to 1 V, appear to be well suited to drive trilayer strips based on CPs that actuate over a similar range. The actuator used, based on polypyrrole (Ppy), gave reliable and repeatable mechanical behaviour for about 50 h. Two sensor–actuator systems have been trialled. In one, the output from a lead-oxygen sensor was electronically augmented to drive a conducting polymer trilayer actuator valve. The desired oxygen concentration was easily maintained with response times of tens of seconds. The other system used the output from a zinc–air cell directly to drive an actuator valve that limited the diffusion of air into the enclosure. Positive oxygen concentration control was achieved but at lower rates of oxygen flux and longer response times. Good sealing of the orifice at closure was found to be critical. © 2004 Elsevier B.V. All rights reserved. Keywords: Oxygen; Sensor; Actuator; Conducting polymer; Fruit; Atmosphere

1. Introduction Improvements to the operating lifetime of bimorph actuators based on conducting polymers (CP) open up some interesting possibilities for practical devices including the prized goal of creating artificial muscles [1–7]. Here we investigate their use in an integrated oxygen control system. The starting point is the possible synergy between electrochemical oxygen sensors, with output potentials up to a volt, and conducting polymer actuators, which change their oxidation state over a similar range. The practical focus chosen for this work was control of the oxygen atmosphere of a “package”, for example a sealed container for the storage of fruit such as apples. It is known that the optimum storage condition for apples is approximately 5% oxygen at a temperature several degrees above zero. Under these conditions the fruit continues to respire, but at a low rate [8]. The fruit industry has considered the use of films of controlled permeability in which the diffusion flux of oxygen in the storage volume matches the respiration requirements. ∗ Corresponding author. Tel.: +64-4-9313202; fax: +64-4-9313754. E-mail address: [email protected] (M.L. Jansen).

0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.03.006

Unfortunately, the respiration rate and the diffusion coefficient of the packaging are both temperature dependent, and it is generally not possible to provide a match over the temperature range that the product will encounter. Thus the level of oxygen in the storage atmosphere will fluctuate. An active valve system which closes when the concentration exceeds the set point, and opens at concentrations below it, would provide a solution. Obviously such a system could be constructed from conventional components, but the value of goods such as fruit can warrant only a simple, low-cost device, and one which could ideally be manufactured as an “intelligent patch” on the packaging component. Mackereth [9] has described a lead cell for the measurement of oxygen concentrations in fluids, which exhibited stability over a lifetime of many months. More recently, similar cells of the kind made by Alphasense [10,11] have been developed and used extensively in the area of safety monitoring. Such cells have admirably fast response (typically a few seconds), but are expensive. Zinc-based air cells [12] are more attractive because they produce a higher voltage than lead cells (1.4 compared to 0.8 V in normal atmospheres), and in food-based applications would be more acceptable. They are available very cheaply in the form of button cells for

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appliances such as hearing aids. Thus, rather than fabricating a custom cell, we elected to investigate the possibilities in this work of using a commercial zinc–air battery as a sensor. Piezo bimorphs provide a possible low power actuator capable of closing an orifice. However, calculations shows that voltages well in excess of 10 V would be necessary to provide a movement of millimeters on a thin film such as polyvinylidene fluoride (PVDF) [13]. In such cases a suitable match of voltage requirements to sensor output could only be achieved with an electronic interface. We therefore concentrated efforts on bimorphs activated by the volume changes which occur when conducting polymers undergo oxidation or reduction. CP systems such as polypyrroles (Ppy) are known to undergo reversible oxidation/reduction processes at potentials <1.00 V versus AgCl. See equation below.

As the polymer changes its oxidation state (between states I and II) charge compensating anions (A− ) are included/occluded from the polymer backbone and significant changes in volume occur resulting in bimorph movement.

2. Experimental 2.1. Oxygen sensors and measurement An O2-A1 cell (10, Alphasense Ltd., UK) was used to measure the oxygen level in the test chambers: the current output from this cell was fed through a current-to-voltage converter and logged (ADC16, High Resolution Data Logger, Pico Technology). In one system this cell was also used indirectly (involving electronic augmentation) to drive the actuator. In a second system (direct sensor–actuator connection) an Energizer AC675 (Eveready Battery Co) zinc–air cell was used and the voltage output used directly as a measure of the oxygen concentration and to drive the actuator. In order to maximize cell life, the zinc–air cell was operated with three of the four diffusion holes blocked off. 2.2. Oxygen flux measurements Preliminary trials included measuring the effect of using different size orifices and diffusion tube dimensions on the flux of oxygen from normal atmosphere at approximately 20% to one at 5% concentration. Measurements were also made of the actual oxygen consumption of apples by monitoring the fall in oxygen in a sealed container containing 15 kg of apples. Oxygen concentrations were measured using the Alphasense A1 cell with

its lead wires sealed through the container wall. Before initiating these measurements nitrogen gas was flushed through the container to generate a starting oxygen concentration of 5%. 2.3. Actuator construction The key component of the actuator is a bimorph cantilever strip: a symmetric trilayer of electrolyte saturated layer coated on both sides with conducting polymer. To one end of the strip (approximately 5 mm × 30 mm) was attached a lightweight plug capable of blocking an orifice. The trilayer was fabricated as described by Sansinena and Olazabal [4] using porous polyvinylidene fluoride sheet (Millipore, 0.45 ␮m pore size, 110 ␮m thick) as the base layer. This was sputter-coated with platinum (0.1 ␮m) on both sides then electrodeposited with polypyrrole from a solution in propylene carbonate (PC) of pyrrole (0.06 M) and tetrabutylammonium hexafluorophosphate (Bu4 NPF6 ) (0.05 M). Approximately 0.5% water was also added to this solution to aid deposition. The counterelectrode in the electrodeposition cell was stainless steel gauze placed on both sides of the vertically suspended platinized PVDF sheet. The whole cell was wrapped in ‘Parafilm’ to exclude air and to limit condensation. It was placed in a freezer at −20 ◦ C for the electrodeposition process. A DC current of 0.05–0.1 mA/cm2 was applied to each side of the cell for 24 h. In this way a polypyrrole coating approximately 30 ␮m thick was obtained. Following electrodeposition the film was rinsed several times with PC to remove unreacted monomer. The area of actuator film, typically a rectangle 25 mm × 5 mm, required for use was cut from the bulk sample, using a sharp blade, ensuring that the two Ppy sides were not pressed together or otherwise made to contact each other. It was then soaked in a 0.25 M solution of Bu4 NPF6 in PC for several minutes, removed from solution and lightly contacted with clean tissue to remove any surface liquid. In use the actuator strip was clamped at one end between platinum contacts. Practical comments on the design and performance of this type of actuator have been made by Madden et al. [3]. The platinum contacts worked reliably, but the contact areas showed discolouration when other metals were used indicating that unwanted reactions were occurring. Prior to use, actuator strips were soaked in propylene carbonate and wiped dry. Tests were done to correlate electrical performance with their deflection. 2.4. Test enclosures and valve assemblies Small test enclosures were constructed with clear perspex tops so that movement of the actuator was visible. The Alphasense oxygen sensor was sealed into the enclosure base, entry and exit ports for nitrogen and air were installed in the enclosure sides, and the actuator was mounted as required according to the two systems described below.

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Fig. 1. Test enclosure for Sensor–actuator system 1: electronic augmentation. The enclosure is maintained at 5% oxygen by separately adding nitrogen and allowing air to enter controlled by the actuator valve. Oxygen concentration is monitored by a lead-based oxygen sensor mounted in the base of the enclosure. The output from this sensor is also used to drive the actuator via simple electronic augmentation. In response to a change of less than 1% oxygen the actuator closes within a few seconds.

2.4.1. Sensor–actuator system 1: electronic augmentation System 1 experiments were conducted in a vessel shown in Fig. 1 in which oxygen consumption was mimicked by the introduction of a nitrogen flow. The natural diffusion of oxygen from ambient into the vessel was augmented by a small stream of air directed to the valve from the outside. In this way the response times of the system were greatly reduced. The Alphasense cell operating in current mode provided a fast response and was used both to monitor the oxygen level and provide a control voltage for the actuator. The latter normally began to close at a voltage of approximately 0.4 V. The output from the oxygen sensor was electronically modified to produce a voltage falling from +1 V at zero oxygen level (to open the valve completely) to −1 V at 10% (to close the valve completely). The valve was nominally closed at 5% concentration. The position of the valve orifice was adjustable making it possible to change the set point. The cantilever actuator was mounted on the inside wall of the enclosure with the long axis bending horizontally. To the free end of the cantilever was attached a Styrofoam ball designed to seat evenly within the orifice. 2.4.2. Sensor–actuator system 2: direct connection (no electronic augmentation) An enclosure was constructed in which the passive oxygen absorption was simulated by using a commercial oxygen scavenger (“Ageless” Oxygen Scavenger) supplied as sachets. The Energizer AC675 cell was sealed into one wall of this enclosure. This cell operated into a load resistor of 300  to force the response into a quasi-voltage mode, and was connected directly across the CP actuator. As for system 1 the Alphasense cell was used independently to measure the oxygen level inside the test chamber. The port allowing

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Fig. 2. Test enclosure for Sensor–actuator system 2: direct connection (no electronic augmentation). The enclosure is maintained at 5% oxygen by using an oxygen scavenger inside the enclosure and allowing air to diffuse in through the orifice controlled by the actuator valve. The actuator is powered directly by the voltage generated by a zinc–air cell responding to changes in oxygen concentration. Oxygen concentration is monitored separately by a lead-based oxygen sensor mounted in the base of the enclosure.

entry of the air was controlled by a cone flexibly suspended to the cantilever actuator mounted on top of the enclosure (see Fig. 2). This gave a more graduated closing action than the hemispherical plug, and to some extent provided self-correcting realignment during closure. A better shut-off was thereby obtained. The use of nitrogen flows to mimic oxygen consumption, as used in system 1, caused an additional flux out of the valve and this interfered with the slow inwards diffusion of oxygen. Instead oxygen consumption was achieved in this system by the use of sachets of a commercial absorbent (Ageless Oxygen Scavenger, Cryovac-Sealed Air (NZ) Ltd., Hamilton).

3. Results 3.1. Oxygen flux The measured consumption of oxygen by apples was 5.4 ml/h kg at 20 ◦ C and 5% oxygen concentration. This is similar to the expected value: Table 1 shows independent data on the respiration requirements of apples (oxygen absorption) in controlled atmospheres. From these measurements it is calculated that a package of 10 kg fruit in a low oxygen environment requires between 5 and 40 ml/h of oxygen, depending on temperature. Table 1 Typical rates of absorption of oxygen in air by apples Temperature (◦ C)

0 20

Rate of O2 absorption by apples (ml/h kg) O2 concentration in air about 20%

O2 concentration in air below 5%

1.3 10

0.5 4

Data from McDonald [14].

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3.2. Sensor behaviour Electrochemical oxygen sensors have a diffusion barrier which allows a flux of oxygen proportional to the external concentration to reach the active electrode; all of this oxygen reacts to produce charge and the internal concentration of gas is zero. A conventional measurement of this reaction rate (via the short-circuit cell current) gives a linear measure of the external oxygen level. However, the CP actuators require a potential of the order of 0.4 V to operate (see Section 3.3). When used as a battery (i.e. close to open circuit conditions) the concentration of oxygen inside an air cell is approximately that of ambient. Under these conditions, the cell voltage is only weakly dependent on oxygen pressure via the Nernst equation; for example a tenfold reduction in ambient concentration from 20 to 2% causes less than 100 mV change in voltage—a value too small to change the state of the actuator. However, the actuators themselves exhibit a leakage current and therefore present a finite load resistance to the sensor. Bearing in mind that the actuator will be draw current from the sensor, the output voltages of the Alphasense and Energizer cells were measured as a function of oxygen partial pressure for different load resistors. The object was to explore operating modes intermediate between short circuit and open circuit. With a restriction in the rate at which oxygen can enter the cells (built into the Alphasense cell; and achieved by blocking three of the four air vents in the Energizer cell), and by controlling the rate at which the oxygen reaction occurs (by the size of the load resistor), oxygen within the cell builds to a value intermediate between zero and ambient. It was believed that this might produce a useable concentration–voltage response. The results are shown in Figs. 3 and 4. The Alphasense cell loaded by 1 k does not reach the 0.4 V target at the oxygen levels of interest, and continues to show a linear current-to-oxygen transfer function. With a 10 k load, actuator operating voltages are in fact reached by 4% oxygen concentration, but with a cell open circuit voltage of only 0.72 V at 20% oxygen concentration, the slow voltage increase for levels above 4% is not satisfactory for control without electronic intervention. The Energizer cell performance is basically similar, but it has a lower internal impedance. A load resistor of 300  gives an output voltage which rises satisfactorily over the actuator operating region, and because of the greater open circuit voltage of the zinc cell, the voltage available continues to rise significantly above 0.4 V as the oxygen level increases (Fig. 4).

0.6

1k load

Alphasense Cell

10k load

0.5

loaded voltage

An orifice 6.5 mm diameter and 11 mm long was found to diffuse 60 ml/h from atmosphere to an internal volume where the concentration was 5%. An orifice as open as this presents problems, particularly with regard to its susceptibility to draughts.

VOC = 0.72V

0.4

Time constant ~ 30 min 0.3 0.2 0.1 Time constant ~ 60 sec 0 0

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Fig. 3. Response of the lead-based Alphasense oxygen sensor under loads of 1 and 10 k, with different time constants as shown. This cell was used to monitor oxygen concentrations and also in system 1 to indirectly drive the actuator.

The time constants of both sensor cells loaded to give a useful output voltage is of the order of 30 min. In the application under consideration, such a slow response is quite acceptable. A feature of the Energizer response is that, unlike the purpose-designed oxygen cell, the current does not fall to zero for zero oxygen concentration. Under these conditions, the device operated as a cell with an open circuit voltage of 160 mV, and 40  impedance. The effect is to elevate the oxygen response on a small voltage pedestal. In these experiments this presented no problems. The zinc–air cell, operating into a load resistor, can therefore provide a voltage output capable of operating an electrochemical actuator at the oxygen levels in question, though the time response is slow. Faster operation requires the cells to work in current mode, which necessitates electronic modification of the sensor signal to drive the actuator. 3.3. Actuator behaviour Capacitor-like charge storage accompanied by simultaneous bending was obtained for freshly prepared actuators; moreover, the oxidation, charge storage and deflection 1.2

Energizer AC675 Loaded cell voltage, V

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1 Time constants ~ 30 min

0.8

600 ohm 300 ohm

0.6

10 ohm 1 ohm 100 ohm

0.4 0.2 0 0

2

4

6

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Oxygen concentration, %

Fig. 4. Response to oxygen of the energizer zinc–air cell, AC 675, under different loads (1–600 ). The time constant at 300  is about 30 min. This output from this cell was used in system 2 to drive the actuator directly.

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Current, mA

10

charge discharge

1 0

100

200

300

400

0.1

0.01

Time, s

Fig. 5. Charging and discharging behaviour of a strip 20 mm × 5 mm of Ppy CP in response to voltage step of 0.6 V and subsequent removal. The discharge current is that measured in small external resistance. The near exponential fall with time for each curve is consistent with the actuator acting as a capacitor with a large internal resistance.

began when potential differences of less than 1.00 V were applied. Significant deflections were obtained at voltages of 0.4 V, reaching a maximum for potentials close to 0.9 V. Fig. 5 shows the charging and discharging characteristics of a polymer strip 20 mm × 5 mm in response to the application of a voltage step of 0.6 V and its subsequent removal. The charge cannot be removed instantly by short circuiting the device. The discharge characteristic shown here is the current flowing through a small external resistance. Both curves show quasi-exponential behaviour, but with quite different time constants, The charge time constant is 35 s and the discharge 100 s. Incremental increases in voltage produce similar charge current curves, but these peaks cease after a voltage of 0.9 V is reached, consistent with all material having been oxidized. These material-dependent time constants limit applications to those with characteristic times of perhaps a minute or slower; oxygen control is an example. Upon charging to equilibrium, there is a residual current corresponding to an ohmic leakage of some 5 k/cm2 . It has been shown previously that such a resistance converts

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the output of an oxygen cell from current output to a useful voltage variation. The leakage resistance therefore can be used to advantage. In the new condition, the actuator behaved as a proportional element, i.e. one with a monotonic relation between applied potential and deflection, which is desirable for control. Actuator performance degraded over a period of 2–3 days. Initially the end of the actuator strip moved at speeds of 5 mm/s in response to step voltage changes, but after 24 h the deflection began to significantly lag the charging current. Finally, the charge storage itself decreased and the deflections ceased. This is almost certainly due to loss of propylene carbonate, since cantilevers could be rejuvenated by rewetting. Weighing tests showed that in ambient air, 90% of the propylene carbonate load in an actuator evaporated in 3 days. 3.4. Sensor–actuator system 1: electronic augmentation Fig. 6 shows the control achieved in the electronically augmented system. The lower trace is the control signal for the actuator, derived from the Alphasense cell, while the upper trace is the actual oxygen level within the cell. Twice in the course of the record, the oxygen supply was increased temporarily, and the resulting traces show that the valve returned control to the desired set point. In terms of control theory, the oscillatory response means that the system has more than one time constant (e.g. one related to the behaviour of the actuator material, the other to the gas fluxes and mixing to alter the concentration measured). The fact that the level settles to a set value is a consequence of the fact that the actuator has a proportional response to the applied voltage, not a “bang-bang” response characterized by the valve opening and closing abruptly at set voltages.

Oxygen level, %

10 8 6

Control Voltage

4 2 0 0

200

400

600

800

1000

-2

Time, s Fig. 6. Control of oxygen concentration with sensor–actuator system 1. The upper trace is the oxygen concentration as measured by the Alphasense sensor cell; the lower trace is the control signal derived from the Alphasense sensor and supplied to the actuator. In the record shown, the oxygen supply was temporarily increased at 100 and 550 s; subsequent traces show that the valve has brought the oxygen concentration back to the desired set point.

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M.K. Andrews et al. / Sensors and Actuators A 114 (2004) 65–72 0.0015

0.0002

0.7

2.5

0.6

2 0

(a)

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0.5 5000

Time, s

(b)

0.001 0.0001 0.0005 0

0 0

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Actuator change, V/sec

3

Concentration change, %/sec

0.8

sensor/actuator voltage

Oxygen concentration,%

3.5

-0.001 -0.0015

-0.0002

Time, s

Fig. 7. Control of oxygen concentration with sensor–actuator system 2. In this system the zinc–air cell powers the actuator directly. (a) Changes in oxygen concentration after it was reduced to approximately 3% by nitrogen flushing, and then left for 5000 s. The oxygen sensor (rapid response) and output from the zinc–air cell (much slower response) more or less tracked each other. (b) Changes in the rate of change of oxygen concentration by taking first derivative of data shown in (a). This shows the relatively slow response of the zinc cell and the movement to an equilibrium oxygen concentration toward the end of the experiment.

3.5. Sensor–actuator system 2: direct connection Fig. 7 shows the results obtained with the sensor–actuator system 2. In this system, with direct sensor–actuator connection, the time constant is necessarily of the order of tens of minutes because of the slow response of the sensors working in voltage mode. Fluxes of gas into the chamber must be low enough that large concentration changes cannot occur in times scales less than this. Fig. 7(a) shows the results of a test in which the chamber oxygen level was reduced to approximately 3% by nitrogen flushing, and then left for 5000 s. The rapidly-responding oxygen sensor and the slower-responding zinc cell output, which is driving the actuator directly, approximately track each other. For the first 200 s, the oxygen level is rising, but this trend is reversed for the following 1500 s as the valve closes. This fall must be produced by the absorption of oxygen within the chamber since any oxygen-rich leaks from the outside can only produce increases in the internal oxygen concentration. Fig. 7(b) emphasizes the control exerted by showing the rates of change of both oxygen concentration and the zinc cell-generated control voltage. The slower response of the zinc cell is clearer, as is the fact that the rate of change of the oxygen concentration is settling toward zero at 5000 s.

4. Discussion The CP material operated as a cantilever seems to be the only substance which offers the possibility of actuation at electrochemical voltages. In its fresh state its performance was quite repeatable in qualitative terms, but marginal for what might be termed engineering precision. The inherent difficulty with the CP material is the incompatibility between the size of the movement needed and the force available. Large displacement is easily achieved, but

the CP material exerts little force in bending. Experiments in which the cantilever was used simply to close a hole in a plate were unsuccessful because it proved impossible to force the polymer to conform reliably to the plate surface. Despite the lateral symmetry of the actuator, small but significant sideways twists occurred which prevented tight closure of the orifice. (Some of this behaviour can be related to variations in CP thickness and other imperfections in the trilayer.) Actuator lifetime in air was reasonably good but well short of that required for an application. It is unclear whether electrolyte evaporated from the exposed edges of the trilayers, or through the coated faces. The demands of operating CP actuators in a liquid environment are well recognised [3]. Newer electrolyte materials such as ionic liquids should be a means of overcoming this problem. It has recently been shown [15,16] that the use of ionic liquid electrolytes with CP systems results in dramatic improvements in shelf life and stability to electrochemical cycling. Galvanic cells are normally operated in short-circuit mode when measuring oxygen. Here we have used a finite resistance shunted across the cell to generate a voltage monotonically related to oxygen concentration. This has to be beneficial to cell lifetime, since smaller currents flow for a particular gas concentration. We have made use of the finite leakage current in the actuator to provide part of this load. For most CP applications, leakage is undesirable. Here this is not the case. An understanding of leakage mechanisms, and an ability to control them would enable a designed load resistance to generate appropriate voltages. The power drawn will affect cell lifetime, and will determine the capacity of the cell required. The development of screen-printed sensor cells would add design flexibility, because it would be possible to connect cells in series and increase the voltage available. In effect, better impedance matching between sensor and actuator would be possible.

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It is useful to compare the oxygen requirements of a zinc–air cell and a hypothetical 10 kg of fruit, which has a maximum requirement of 40 ml/h (11 × 10−3 ml/s). The Energizer cell can supply 2 mA continuously. The cathode reaction is: O2 + 2H2 O + 4e ⇒ 4OH− At 2 mA, the oxygen requirement is only 0.11×10−3 ml/s. The packaging valve must be able to supply 100 times the oxygen diffusing through the entry holes of the button cell. The Alphasense oxygen monitor is supplied by diffusion through a capillary, and uses 1000 times less than the fruit package. Given the amount of oxygen required, the valve openings had to be several millimeters in diameter, which made them susceptible to local air currents. Tubular orifices, by providing isolation from external air currents, would be valuable but appear to be incompatible with the consumption requirements. Expressions for the diffusion of a gas through another species in a tube are well established [17,18]. Taking the diffusion coefficient of oxygen in nitrogen as 0.1 cm2 /s, a diffusion flow of 40 ml/h can be shown to require an area to length ratio of 7.3 × 10−3 m. Table 2 shows the implied dimensions of various tubes which fulfill this requirement. Only the impossibly large combination to the left of table approaches tube-like geometries for which the formula applies. The dimensions at the right approach “hole in wall” dimensions and the formula used is not appropriate for such geometries. Experiments showed that the fluxes achieved by an orifice of the latter type, e.g. a diameter of 3 mm in a wall of 1 mm are almost a factor of 10 below those predicted in table. Accordingly, the valve orifice must be further increased, but in doing so the more open valve loses protection from draughts. Further, very good closure of such valves was necessary to change the flux through them significantly. Measurements with the ball closure of the port in Fig. 1 (6.5 mm diameter by 11 mm long) showed that the flux for the port fully open was 60 ml/h, reducing to 23 ml/h when “closed” by the actuator. Much better seals were necessary to improve significantly on this, which was why the suspended cone design was used in the electronic-free version. Clearly, a practical device to control oxygen concentrations would require a better means of closure; alternatively the problem of influx rates being susceptible to ambient air currents may be solved by the use of plugs of filter-like material in or about the orifice. Table 2 Range of dimensions for a tubular orifice able to supply oxygen by diffusion in nitrogen at 40 ml/h and with area to length ratio of 7.3×10−3 m Orifice dimension (mm) Length, L Diameter, d Approx., L/d

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An integrated sensor–actuator of the kind described here, using a conducting polymer based actuator to effect orifice closure, has been behind the present work. At present the major difficulties of such a system appear to be reliability of closure of the orifice, actuator lifetime and the low voltage and power available from a metal–air electrochemical sensor. Progress on these matters will be necessary before the goal of a practical low-cost gas control system can be fully realised. 5. Conclusions An actuator based on a conducting polymer trilayer showed a reliable and repeatable mechanical behaviour for about 50 h, after which time its performance deteriorated. This was believed to be due to the evaporation of propylene carbonate. A practical longer lived actuator would require an improved electrolyte, or a means of encapsulation to prevent evaporation. Platinum pressure contacts to the trilayer worked satisfactorily, with no evidence of electrochemical action. This was not the case with other metals. In order to supply oxygen fluxes to packages of approximately 10 kg of respiring fruit it was found that diffusion ports several mm in diameter were required. Such ports could be closed by the actuator devices described here. While the trilayer bimorph strip actuator generally performed well, it was not well suited to a simple flap valve design. Lightweight plugs and cones attached to the cantilever strip were required to achieve adequate closure. Used with interfacing electronics, a conventional EC oxygen sensor and an actuator easily held the atmosphere inside a container to a desired oxygen level. This is readily achieved with an electronic interface, which enables response times to be tens of seconds, but is also shown to be have been achieved in an electronic-free manner by direct coupling between the sensor cell and the actuator. Electrochemical actuators behave electrically as leaky capacitors. Thus, either the leakage resistance, or the leakage augmented by further leakage resistors, was able to force the output of a zinc–air cell into an output voltage rising approximately linearly with oxygen concentration. Employing only such an electrochemical oxygen sensor and a conducting polymer actuator, control of oxygen with a small volume was demonstrated. In principle, wholly electrochemical sensor/actuator patches could be made by screen printing methods and fixed into packaging material for atmosphere control. However, aside from obvious questions of lifetime, achieving dynamic repeatability of the CP material to ensure reliable valve closures was the most limiting issue encountered here. Acknowledgements

1000 96 10

100 30 3

10 10 1

1 3 0.3

The assistance given in this project by Jason Morris and H. de Ruiter, IRL, Communications and Sensors Team, is

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gratefully acknowledged. The O2-A1 oxygen sensor was kindly donated by John Saffell of Alphasense Ltd., UK. The work was undertaken with the financial support of the New Zealand Foundation for Research Science and Technology. Prof. Wallace and Assoc. Prof. Spinks acknowledge the continued support of the Australian Research Council.

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Biographies Michael K. Andrews received an MSc from Auckland University and a PhD in space physics from Imperial College. He subsequently worked in the field of silicon fabrication, particularly in the development of sensors. This included work on electronic noses based upon conducting polymer chemiresistors. He retains an interest in microfabrication, but for the last several years has been developing industrial instrumentation for measuring the physical properties of plantation-grown timber. Murray L. Jansen received his MSc degree from Victoria University of Wellington and PhD in physical and analytical chemistry from the University of Calgary in 1974. Within the Communications and Sensors Team at Industrial Research he has worked on pH ISFET technologies and more recently on gas sensors utilising conducting polymers, conducting composite materials and QCM devices. Geoffrey M. Spinks has B Applied Science (1985) and PhD (1990) from the University of Melbourne. He is currently associate professor at University of Wollongong where his research areas include the development of polymer materials for sensors and actuators and for use in paint and adhesive systems. Particular interests are in the characterisation of mechanical properties of polymers, especially under dynamic control. Dezhi Zhou received his BE and ME degrees from Tsinghua University, Beijing and PhD in chemistry from the University of Wollongong, Australia in 1997. He is currently research fellow at the Intelligent Polymer Research Institute at the University of Wollongong, with research interests in conducting polymer electromechanical systems, pressure sensors and transducers, thin film fabrication and water-soluble conducting polymer processing. Gordon G. Wallace is Director of the Intelligent Polymer Research Institute, Wollongong, Australia. He has published more than 300 refereed articles mostly in the areas of inherently conducting polymers and more recently carbon nanotubes. His research involves the synthesis and characterisation of novel conducting polymers based on polypyrrole, polyaniline and polythiophenee. Fundamental studies under his supervision investigate how these “intelligent” materials interact with their chemical and physical environment. Prof. Wallace currently holds an Australian Research Council (ARC) professorial fellowship.