- Email: [email protected]
Inherently conducting polymer modified polyurethane smart foam for pressure sensing
Sensors and Actuators A 119 (2005) 398–404
Inherently conducting polymer modified polyurethane smart foam for pressure sensing Sarah Brady, Dermot Diamond, King-Tong Lau ∗ National Centre for Sensor Research, Dublin City University, Dublin 9, Ireland Received 5 March 2004; received in revised form 14 October 2004; accepted 16 October 2004 Available online 18 March 2005
Abstract A new compressible conducting material has been developed by coating polyurethane (PU) foam with inherently conducting polypyrrole (PPy). The optimised conditions for preparation of the conducting foam have been investigated. Evaluation of the conducting foam shows that a linear relationship exists between the conductance and the stress applied. Parameters such as sensitivity, dynamic range, repeatability of this pressure sensor are discussed. The use of this soft pressure sensor in a prototype breath monitor is also reported. © 2004 Elsevier B.V. All rights reserved. Keywords: Polyurethane; Pressure sensing; Polypyrrole
1. Introduction Inherently conducting polymers such as polypyrrole and polyaniline are often referred to as a “synthetic metals”, which possesses the electrical and magnetic properties of a metal, while retaining the mechanical properties of a polymer. Active research has been carried out to investigate the application of these materials in corrosion protection, rechargeable batteries, electrochromic displays, conducting composite materials, biosensors, chemical gas sensors, actuators, microextraction platforms, electronics, electrochemical energy sources, optical devices and smart fabrics [1–5]. These conducting polymers are generally insoluble and brittle and are difficult to process, therefore limiting their use. Chemically modifying the monomers with a substituent or using a large organic dopant has successfully improved the processibility of these materials [6,7], producing thin films and ultra thin films of substitute polyanilines fabricated by Langmuir–Blodgett technique [8].
∗
Corresponding author. Tel.: +353 1 700 7000; fax: +353 1 700 8021. E-mail address: [email protected] (K.-T. Lau).
0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.10.020
One interesting area of research based on conducting polymer has been the development of smart textiles and wearable electronic devices. Conventional smart fabrics are made by weaving metal wire into fabrics, which combines with small electronic components, sensors (chemical, electrochemical or optical) and circuitry to produce smart (metal-based or MB) wearable garments [5]. Many of the devices available for this type of sensing rely upon electronic components attached to the material [9] or are not mobile with the wearer [10]. A more innocuous approach is to generate smart fabrics by directly coating conducting polymers onto a substrate material e.g. LycraTM [11], hence reducing the use of metal component within the fabrics. The major advantages of using conducting polymer (C-P)-based fabrics are that they retain the natural texture of the material and the fabric can be processed as normal. These materials normally work as strain/pressure gauge and find applications in wearable medical monitoring systems (e.g. for limb movement) for clinical use [12]. In sports applications, performance statistics of an athlete, such as heart rate and breath volume, can be monitored by noninvasive means using equipment such as cardiotachometers and spirometers. Much research focuses on monitoring and correcting the amount and spatial distribution of pressure to
S. Brady et al. / Sensors and Actuators A 119 (2005) 398–404
reduce the risk of repetitive strain injuries (RSI) and noncontact ligament injuries. It is known that the conductivity and optical properties of conducting polymers such as polyaniline (PANi) and polypyrrole (PPy) depends on the oxidation state of the main chain as well as the degree of protonation of nitrogen atoms in the polymer backbone in PANi [13]. The physical properties of the coated fabrics depend on the nature of the monomer, the dopant, the substrate and the thickness of the coating employed. It is therefore possible to tailor parameters such as the conductivity and the hydrophobicity of the fabrics by choosing appropriate monomers and reaction conditions. However, existing C-P-based smart fabrics are typically thin pieces of conducting polymer coated textiles that work as two-dimensional strain gauges, i.e. they have to be stretched to give a change in conductivity and are not sensitive to force normal to the planar surface of the fabric. In this study we report the synthesis and properties of a new class of C-P-based smart fabric prepared by chemically coating polyurethane foam with conducting polymer (polypyrrole). These materials are soft, compressible and versatile and, in contrast to coated textiles, are sensitive to forces from all three dimensions. These foams are porous materials with large surface area to bulk ratios, have excellent fluid absorbance properties, and are robust to repeated compression and expansion. They could form the transducer basis for many sensing applications, both as physical transducers, and as chemical sensors, where the C-P coating properties are affected, for example, by certain gases or liquids.
2. Experimental 2.1. Chemicals and materials Pyrrole, naphthalene di-sulphonic acid (NDSA), and ferric chloride were obtained from Aldrich (Dublin, Ireland). Pyrrole was distilled prior to use. The NDSA, and ferric chloride were used without further purification. MilliQ water was used as the solvent for polymerisation and washing. The polyurethane (PU) foam substrate, was obtained from IRETEX (Leixlip, Ireland) and was first washed with soapy water and then rinsed with excess MilliQ water and dried in air prior to use. 2.2. Smart foam synthesis The conductive foams were prepared by in situ chemical polymerisation of the appropriate monomer, i.e. pyrrole, to retain the elasticity present in the foam substrate. The chemical polymerisation of pyrrole was modified from the method used by Oh et al. [11]. A 300 ml solution containing 0.04 M pyrrole and 5.4 mM NDSA was stirred in a 500 ml Duran borosilicate glass dish. A piece of polyurethane (10 cm × 10 cm × 1.7 cm), was soaked in the above solution for 2 h, ensuring that the foam was completely wetted with the
399
pyrrole/NDSA solution. Three hundred milliliters of 0.04 M FeCl3 was added and the mixture was stirred regularly for 2 h at room temperature and then allowed to stand overnight. The black PPy–PU foam was then removed from the container and triturated with MilliQ water to remove any loose bound PPy and blotted dry using tissue paper before being placed in an oven at 40 ◦ C overnight. The weight and the electrical conductivity (σ) of the PPy coated foam were then recorded. This coating procedure was repeated an additional three times to produce the final smart foam. 2.3. Conductance measurement 2.3.1. Sample preparation The PPy–PU foam was cut into specimens with dimensions of 1.7 cm × 1.7 cm × 1.3 cm. Conductive self-adhering foil (Radionics, Dublin, Ireland) was used to connect the two opposite end of the foam to the HP 34401A constant current multimeter (Hewlett Packard, Ireland). The data was collected by a PC using the software supplied by the manufacturer (HP multimeter software Version 1.1). 2.3.2. Experimental setup Three methods for determining the effect of pressure on the foam sample were used. The schematics of the setups used are given in Fig. 1A and B. The first setup (Fig. 1A) was made up of two PMMA platforms. Four metal pins (L = 10 cm, D = 0.2 cm) were fixed to each corner of a bottom platform (10.0 cm × 5.0 cm × 0.6 cm) so that there was 3.0 cm separation between the pins. Four holes (D = 0.21 cm) were drilled into the upper platform (5.5 cm × 6.0 cm × 0.2 cm, 6.203 g) so that it could thread through the four pins. The purpose was to give stability to the weight placed on the centre of the top platform. The sample prepared from above was put in between the two platforms and secured to the bottom platform by using two pieces of self-adhesive copper tape, separated by a distance of 1 cm. When known weights were placed onto the upper platform, the foam was compressed and the change in resistance was measured by the HP multimeter. The second setup, as shown in Fig. 1B, employed a clamp to sandwich the foam sample, again using self-adhesive copper tape as contact points between the sides of the foam and the multimeter. The clamp was gradually tightened to compress the foam sample from an initial length of Li to a final length of Lf . This produced a change in the resistance in the C-P coated foam that was measured by the multimeter. All experiments were run at room temperature and atmospheric pressure. The stress versus conductance profiles were obtained with an InstronTM machine (model 4202). The data was collected using the Series IX software as supplied by InstronTM , which recorded the load acting on the crosshead at a rate of 10 data pt/s while simultaneously monitoring the conductance and the length of the sample. The rate of loading and unloading was 2 mm/min throughout the experimentation. The
400
S. Brady et al. / Sensors and Actuators A 119 (2005) 398–404
Fig. 2. Structure of polypyrrole (a) and 1,5-naphthalene di-sulphonic disodium salt, Na2 NDSA (b).
Fig. 1. Schematic of the experimental setups, setup A is for compression study and setup B is for calibrating the foam sensor with known weight.
dimensions of the piston head were 50.44 mm × 32.86 mm. It was important that all the foam samples were cut to a smaller size than this to ensure that an even force was applied across to the entire foam area.
3. Results and discussion 3.1. General properties of PPy coated PU foam The PU foam used in this experiment was a nonconductive, light grey, soft, sponge like material that could be reversibly compressed. When it was coated with polypyrrole it became conductive and black in colour with a gold tint. The tactile properties (i.e. soft and compressible) were retained. The physical stability of the PPy coating on PU foam was found to be excellent. It was able to withstand vigorous washings (rubbing, squeezing) with water. When a piece of PU–PPy was cut through with a scalpel it was seen that the polypyrrole had completely penetrated into the PU matrix, resulting in a completely black mass inside out. The monomer pyrrole molecules were able to penetrate into the PU matrix and the oxidative polymerisation process resulted in a PPy–PU composite rather than simply a PPy coating adhered onto the PU surface. This finding explain the excellent
physical stability of the coating, however, it results in film thickness determination by methods such as SEM impractical because there is no distinctive layers to be seen. The structure of the coating material, i.e. the conducting form of polypyrrole and the dopant anion, naphthalene disulphonic acid is shown in Fig. 2. In general, highly doped polypyrrole contains 0.2–0.4 positive charges per monomer unit depending on the nature of the counter anion or dopant ion (represented here by X− ). These anions are incorporated into the polymer structure to maintain electroneutrality [14]. NDSA was used as a model dopant for PPy as it gives more stable polymer and yields highly conductive product in the order of mS/cm range. Other anions such as chloride, dodecylbenzene sulphonate, p-toluene sulphonate may be used in place of NDSA. A slow drift of baseline conductivity was observed for PPy coated PU foam when left in air therefore the foams were stored in a sealed polythene bag. This drift in baseline conductivity is due to the nucleophilic attack on the polypyrrole rather than due degradation of the polyurethane substrate. The crosslinking found in polyurethane foams makes it resilient to polymer degradation [15]. The nucleophilic attack on the polypyrrole layer gradually oxidises, by attacking the pyrrole ring to form carbonyl groups that break up the polymer backbone rendering it less conducting [16,17]. 3.2. Polypyrrole loading The sequential coating of PU foam with conducting polymers resulted in an increase of the overall weight of the foam and the conductivity of the foam also from being an insulating material to a conductive material (ca. 1.41 mS/cm). Table 1 clearly shows that the conductivity of the modified foam depends on the weight of conducting polymer deposited, which in turn depends on the number of coating layers deposited on to the foam substrate.
S. Brady et al. / Sensors and Actuators A 119 (2005) 398–404
401
Table 1 Effects of polymer loading on conductivity of PU foam, where wt and σ are the weight and resistance of the foam sample No. of coatings (n)
Total weight deposited (g)
Ratio (wtn /wt1 )*
Conductance, σ (mS/cm)
Ratio σ n /σ 1
0 1 2 3 4
0 0.1408 0.2815 0.7037 0.8394
– 1.00 1.99 4.99 5.96
– 0.000578 0.0714 1.25 1.41
– 1.00 123.50 2162.60 2439.40
The subscript n denotes the weight and resistance of the foam material after the nth polypyrrole deposition layer.
The first coating procedure resulted in a very thin polymer coating as most of the conducting polymer was in the form of a fine dispersion in the solvent rather than adhered to the foam. There was only a small gain in weight, with negligible conductance (5.78 × 10−4 mS/cm) being achieved. The second coating procedure added on a similar weight to the amount obtained from the first coating procedure, suggesting that the thickness of the PPy coating has roughly doubled (Table 1, wt2 /wt1 ≈ 2). However, the conductance measured was 7.14 × 10−2 mS/cm, which is two orders of magnitude higher than the single coated foam (Table 1, σ 2 /σ 1 ≈ 123). It is believed that the PPy film thickness (which relates to charge carrier density within the bulk polymer film) is critical for the coated foam to conduct electricity efficiently. After the third coating being put on, a ca. five times increase in PPy weight was obtained compared to the first coating, and resulted in an excellent electrical conductance of 1.25 mS/cm (Table 1). Further coating added produced much smaller PPy weight increases, and small improvements in conductivity. Therefore, we have taken conducting foams with coated with three layers of PPy as the optimised material to be used in all subsequent studies. It has been mentioned that the PPy coating penetrated into the PU matrix and it was not possible to determine PPy film thickness by SEM. A rough mathematical estimation was therefore used to calculate the film thickness from the weight deposited onto the foam based on an estimated surface area. However, because polyurethane foam is a porous material with irregular pore sizes, it is very difficult to even estimate the outer surface area of the foam let alone the overall surface area, therefore the data shown here is only for indicative purposes. The outer surface area of the foam, ignoring the pores and inner porous area, was 268 cm2 , one would expect the total surface area of the foam be much bigger (1–2 orders of magnitude bigger). The total weight of PPy put on after the first coating was 140 mg, and given that the density of PPy is close to 1, the volume of the coating would be approximately 0.14 cm3 . This crude calculation estimates that the first coating gave a film thickness of around 0.5–5 nm. The second coating added on approximately the same thickness to give an approximate 1–10 nm thickness range but with a big increase of conductance. In addition to the thickness increase, it is likely that the second coating lead to a much more homogenous coating of PPy throughout the surface of the foam resulting a big increase in conductivity. The third
coating gave an optimal conducting foam with estimated PPy thickness in the region of 2.5–25 nm. 3.3. Sensor responses The conductance of ICPs, in essence, depends on the density of the charge carrier, i.e. polarons, present in individual polymer chains and the efficiency of charge movement between chains. Hence, the conductivity of a continuous conducting polymer film is described by MacDiarmid [18] as σbulk = f (σintra ) + f (σinter ) + f (σdomain )
(1)
where σ bulk is the bulk conductivity of a conducting polymer, which is a cumulative result of the intra-molecular (σ intra ), inter-molecular (σ inter ), and the inter-domain (σ domain ) conductivities of the polymer film structure. One would expect the conductivity of the PPy coated PU foam also obey Eq. (1). Given that the PU foam contains a large number of empty holes, when a pressure is exerted to compress the foam, it reduces the foam’s overall volume, thus increasing the contacting area of the surface that is covered with PPy film. This in turn enhances the apparent density of the polaron and also shortens the conducting path length (increases magnitude of σ inter and σ domain ) to result in an increase in bulk conductivity. This effect was verified by using the setup shown in Fig. 1A, with which the change in resistance was determined when a foam sample was gradually compressed from an original length L0 to Li . The main feature in Fig. 3 shows two plots of dR (R0 − R) versus L0 /L generated from two repeats of the compression experiment. The use of dR instead of R was to change the plot from a negative slope to a positive slope and to normalise the data presentation. These two plots showed similar linear regions between L0 /L values 1–5 (R2 = 0.99, average slope is 16Ω for n = 2), beyond which the curves began to level off. This observation could be explained by the fact that there was a limit to how much a material can be compressed, when the conducting foam with a length L was compressed to reach Llimit then L0 /L became a constant and hence the resistance R reached a constant value. The maximum resistance change that could be obtained was denoted in Fig. 3 as dRlimit . In this case the value of dRlimit obtained for the conducting foam was approximately 82 k and the Llimit was roughly 1.9 mm from an original length of 1.7 cm (corresponding to ca. 89% reduction in length).
402
S. Brady et al. / Sensors and Actuators A 119 (2005) 398–404
Fig. 3. The resistance change (dR) vs. L0 /L plots obtained from two repeats of the foam compression study. The inset shows a plot of resistance change (dR) vs. % change in length using the same data set as the solid triangle plot shown in the main feature.
The inset in Fig. 3 is a plot of responses versus percentage change of length L. Two distinct regions could be seen where a rather small but steady initial decreases in resistance was observed, followed by much sharper changes after the foam was compressed to approximately 50% of its original length. This observation is easy to understand; initially when the PPy coated foam was compressed the interstitial spaces within the foam decreases gradually with very slow increases in the contacting area between PPy coated surfaces (i.e. air was being squeezed out). When the compression finally reached a critical point (ca. 50% of the original length) where a massive increase in the contact area was achieved and a sharp drop in resistance was observed. Further compression resulted in more intimate contact between PPy chains and a linear change (R2 = 0.99 between 50 and 90% length change) in resistance was observed. The responses of the conducting foam to various weight placed onto the foam (or force applied) was investigated using the setup as shown in Fig. 1B. The weight compressed the conducting foam to reduce the overall length of the material and resulted in an increase in conductivity. This phenomenon is similar to the compression experiment described above. When the conductance G (i.e. the inverse of the electrical resistance) of the conducting foam obtained was plotted against the force applied, linear plots (R2 = 0.98) were obtained for the two repeats shown in Fig. 4. The average sensitivity (n = 2) obtained were 0.0007 mS N−1 . The full dynamic range of response was not investigated because only limited weights could be fitted onto the small platform. The data indicates that the conductance G of the foam is proportional to the force applied onto it. Therefore a simple relationship can be established where G=
1 = kF + C R
(2)
where k is a proportionality constant and C a constant term.
Fig. 4. Plot of normalised conductance (G/G0 ) vs. the stress applied onto a conducting foam sample. Data was normalised by using the ration of the conductance (G) of the material to the baseline conductance of the material (G0 ).
The data shown in Figs. 3 and 4 indicate that the repeatability of the foam sensor was poor. The main reason for this is that when the foam has been subject to intense compression, it results in hysteresis in the PU substrate i.e. loss of internal energy, and it would take a long time (hours) to get back to normal baseline. This phenomenon can be seen in the data obtained with an InstronTM instrument where a foam sample was subject to a steady compression (2 mm/min) followed by a reverse unloading process during which the increase in stress and the change of the length of the foam was monitored. Fig. 5 shows the two consecutive loading/unloading cycles obtained that show the hysteresis effect when the foam sample was subject to an increasing load until the internal energy that maintained the structure of the foam was overcome. The first loading cycle showed an increase in strain with applied stress until a limit was attained. The reverse process showed that a shift of the strain was observed resulting from a loss of internal energy due to the foam had a slower recovery time than the releasing speed of the Instron crosshead. The second cycle resulted in smaller shift in the strain profiles and also that they were closer to the first unloading profile
Fig. 5. Plot of stress (force per unit area) vs. strain (L0 − L/L0 ) applied onto a conducting foam sample obtained by an InstronTM instrument. Upward arrows indicate loading and downward arrows indicate unloading process.
S. Brady et al. / Sensors and Actuators A 119 (2005) 398–404
403
Fig. 6. Normalised (R/R0 ) trace of PPy coated PU foam when repeatedly exposed to a force of 2.30 N.
suggesting that mechanically the foam was in a lower energy state. This unwanted effect may be minimised by limiting the amount of stress applied to the foam sensor because this hysteresis effect is much less significant if a small load (0.813 N/cm2 ) is applied for a short time which allows the PU substrate to recover quickly in ca. 5 s as shown in Fig. 6. Fig. 6 also shows the repeatability of the foam when subject to a small known applied weight. A fixed weight (235 g or 0.813 N/cm2 ) was repeatedly put onto and then taken off from a piece of conducting foam while continuously monitoring the conductivity. The data has been smoothed, using a 5-point moving average smoothing algorithm and normalised by dividing the resistance (R) by the baseline resistance (R0 ) of the sensor. The foam responded rapidly with a drop in resistance when the weight was placed on the upper platform and reached a pseudo-equilibrium within seconds and recovered in approximately 5 s after the load was removed. It can be seen from the trace that the baseline resistance is much noisier than the resistance compressed sample. It was because the foam, being a light-weight sponge like material, is sensitive to external vibration; when the foam was compressed it changes into a denser material and this vibration effect is less significant. Due to the mechanical properties of the PU foam different response characteristics of the PPy coated PU foam subject to different weights were observed. Fig. 7 shows the data obtained by monitoring the response of the conducting foam to three different applied weights that corresponding to values of 9.1, 13.7 and 27.5 N. The results indicate that the initial response to all weights used were very similar with the amplitude of the response dependent on the force applied to the sensor. However, the times to reach equilibrium were different and were found to depend on the weight used in such a manner that bigger applied weight resulted in shorter time to reach equilibrium. When the lightest weight of ca. 9.1 N was applied an interesting respond feature was observed where the weight seemed to have sunk down slowly with the foam structure adjusting to accommodate the weight. It resulted in a response time of ca. 50 s. When the weight applied was increased to13.7 N a rapid response time of ca. 15 s was obtained. The slow weight accommodation feature was barely
Fig. 7. Normalised (R0 − R) trace of PPy coated PU foam as pressure sensor when exposed to different forces.
visible suggested that a force or weight that exceeded the internal energy of the foam would yield a quick response. This hypothesis was proved by further increasing the weight to 27.5 N, which resulted in a rapid respond and it reached a plateau in ca. 10 s. The PPy coated PU foam has been used for developing a breathing monitor, whereby the foam sample is incorporated into a harness to wrap around the ribcage area. The movement of the ribcage during breathing exerts pressure on the conducting foam causing an increase in conductivity of the material. Fig. 8 shows a real time trace which illustrates the repeated movement of the ribcage of a volunteer (SB) during breathing. Initially a deep breathing pattern was observed from a standing posture that showed high amplitude and slow rate. When the subject changed to a sitting down posture and breathed normally a decrease in the amplitude of the signal with an increase in breathing rate was observed. When the subject resume the deep breathing exercise the amplitude of the responses increased and the rate slowed down. This simple experiment shows that the breathing device is able to trace the rate and amplitude of breathing. Therefore it may be useful, with optimised sensor design that incorporated into wearable garment, to monitor breathing of infants or other patients that required special care. The potential of this conducting foam to be used in real life application has been demonstrated. However, there are
Fig. 8. Real time trace of PPy coated PU foam as pressure sensor to monitor ribcage movement while breathing.
404
S. Brady et al. / Sensors and Actuators A 119 (2005) 398–404
limitations concerning both the conducting polymers and the substrate material that are under investigation and these may seriously restrict the practicality of these material. The most important limitations are: (1) drift in conductivity of the conducting foam over time and (2) foam hysteresis after compression. The first disadvantage (1) is completely dependent on the chemical and physical stability of the conducting polymer coating. The cause of drift can be due to environmental interferences such as humidity effects and de-doping of PPy by amines that are present in the atmosphere. Such effects may be prevented if the foam is sealed in an airtight environment. The mechanical properties of the PU substrate are largely responsible for (2). Therefore further research is needed to find conducting polymers that are very stable and offer a compatible surface for ICPs. The identification of substrate foam materials that are robust, and free from hysteresis when repeatedly compressed is also important for the sensor to give reliable results.
[6] P. Audebret, N. Girult, T. Kaneko, Synth. Met. 53 (1993) 251. [7] J.Y. Lee, D.Y. Kim, C.Y. Kim, Synth. Met. 74 (1995) 103. [8] P.J. Kulesza, Proceedings of the Third Baltic Electrochemical Conference, 2003. [9] J.A. Paradiso, K. Hsiao, A.Y. Benbasat, Z. Teegarden, IBM Syst. J. 39 (2000) 511. [10] Sensor Products Inc. See http://www.sensorprod.com/podiascan.html (accessed 27 February 2004). [11] K.W. Oh, H.J. Park, S.H. Kim, J. Appl. Polym. Sci. 88 (2003) 1225. [12] P. Waters, Textile News Online 6, 2002. See http://www.tft.csiro.au/textile news/2002 1q/knowing cloths.html (accessed 27 February 2004). [13] R.S. Kohlman, A.J. Epstein, in: T.R. Skotheim, R.L. Elsenbaumer, J.R. Reynolds (Eds.), Handbook of Conducting Polymers, Marcel Dekker, New York, 1997, p. 85. [14] D. Kincal, A. Kumar, A.D. Child, J.R. Reynolds, Synth. Met. 92 (1998) 53. [15] M. Rutkowska, K. Krasowska, A. Heimowska, I. Steinka, H. Janik, Polym. Degrad. Stabil. 76 (2002) 233. [16] R. Mazeikiene, A. Malinauskas, Polym. Degrad. Stabil. 75 (2002) 255. [17] M.M. Chehimi, E. Abdeljalil, Synth. Met., in press. [18] A.G. MacDiarmid, Polyaniline and polypyrrole: where are we headed? Synth. Met. 84 (1997) 27.
Biographies 4. Conclusion A simple novel pressure sensing material has been developed and calibrated that combines the compressible characteristic of polyurethane foam and the inherent conducting ability of polypyrrole. The conductance of the conducting foam is found to change linearly with the force applied. The application of this conducting foam in wearable sensor has been demonstrated.
Acknowledgements We would like to acknowledge an IRCSET grant for SB RS/2002/765-1 and support from Science Foundation Ireland (Adaptive Information Cluster Award).
References [1] G.G. Wallace, M. Smyth, H. Zhoa, TRAC 18 (1999) 245. [2] M. Vidotti, L.H. Dall’Antonia, S.I. Cordoba de Torresi, K. Bergamaski, F.C. Nart, Anal. Chim. Acta 489 (2003) 207. [3] J. Wu, X. Yu, H. Lord, J. Pawliszyn, Analyst 125 (2000) 391. [4] J.D. Madden, R.A. Cush, T.S. Kanigan, I.W. Hunter, Synth. Met. 113 (2000) 185. [5] M.A. El-Sherif, J. Yuan, A. MacDiarmid, 11 (2000) 407.
Sarah Brady obtained her BSc degree in 2002 in Analytical Science from Dublin City University. She is currently a PhD student in the School of Chemical Sciences at the same University. Her research project involves the development of sensor/actuator systems based on inherently conducting polymer. This smart foam project pioneers the use of conducting foam for pressure sensing and as smart filter for chemical sensing. Dermot Diamond obtained his BSc, MSc, PhD and DSc from Queen’s University, Belfast. He is currently leader of the Adaptive Sensors Group at Dublin City University, Ireland, and director of the Adaptive Information Cluster, a research initiative funded by Science Foundation Ireland. His research interests include molecular recognition; host–guest chemistry; ligand design and synthesis; electrochemical and optical chemical sensors and biosensors; lab on a chip devices; with applications in environmental, clinical, food quality, and process monitoring. More recently, he has focused on the development and applications of fully autonomous sensing devices; wireless sensors; sensor networking; and developing the potential of devices and sensors to provide information for wireless networked systems. K.T. Lau was a graduate of Royal Holloway and Bedford New College (now the Royal Holloway College), University of London. He then acquired a MSc degree in Chemical Research on array sensing with Quartz Crystal Microbalance from Birkbeck College, University of London, where he stayed on to obtained a PhD on the development of amperometric biosensor for glucose measurement. He is currently a senior research fellow in the National Centre for Sensor Research, Dublin City University. His main research area is in sensor development and molecular recognition.
Inherently conducting polymer modified polyurethane smart foam for pressure sensing Sarah Brady, Dermot Diamond, King-Tong Lau ∗ National Centre for Sensor Research, Dublin City University, Dublin 9, Ireland Received 5 March 2004; received in revised form 14 October 2004; accepted 16 October 2004 Available online 18 March 2005
Abstract A new compressible conducting material has been developed by coating polyurethane (PU) foam with inherently conducting polypyrrole (PPy). The optimised conditions for preparation of the conducting foam have been investigated. Evaluation of the conducting foam shows that a linear relationship exists between the conductance and the stress applied. Parameters such as sensitivity, dynamic range, repeatability of this pressure sensor are discussed. The use of this soft pressure sensor in a prototype breath monitor is also reported. © 2004 Elsevier B.V. All rights reserved. Keywords: Polyurethane; Pressure sensing; Polypyrrole
1. Introduction Inherently conducting polymers such as polypyrrole and polyaniline are often referred to as a “synthetic metals”, which possesses the electrical and magnetic properties of a metal, while retaining the mechanical properties of a polymer. Active research has been carried out to investigate the application of these materials in corrosion protection, rechargeable batteries, electrochromic displays, conducting composite materials, biosensors, chemical gas sensors, actuators, microextraction platforms, electronics, electrochemical energy sources, optical devices and smart fabrics [1–5]. These conducting polymers are generally insoluble and brittle and are difficult to process, therefore limiting their use. Chemically modifying the monomers with a substituent or using a large organic dopant has successfully improved the processibility of these materials [6,7], producing thin films and ultra thin films of substitute polyanilines fabricated by Langmuir–Blodgett technique [8].
∗
Corresponding author. Tel.: +353 1 700 7000; fax: +353 1 700 8021. E-mail address: [email protected] (K.-T. Lau).
0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.10.020
One interesting area of research based on conducting polymer has been the development of smart textiles and wearable electronic devices. Conventional smart fabrics are made by weaving metal wire into fabrics, which combines with small electronic components, sensors (chemical, electrochemical or optical) and circuitry to produce smart (metal-based or MB) wearable garments [5]. Many of the devices available for this type of sensing rely upon electronic components attached to the material [9] or are not mobile with the wearer [10]. A more innocuous approach is to generate smart fabrics by directly coating conducting polymers onto a substrate material e.g. LycraTM [11], hence reducing the use of metal component within the fabrics. The major advantages of using conducting polymer (C-P)-based fabrics are that they retain the natural texture of the material and the fabric can be processed as normal. These materials normally work as strain/pressure gauge and find applications in wearable medical monitoring systems (e.g. for limb movement) for clinical use [12]. In sports applications, performance statistics of an athlete, such as heart rate and breath volume, can be monitored by noninvasive means using equipment such as cardiotachometers and spirometers. Much research focuses on monitoring and correcting the amount and spatial distribution of pressure to
S. Brady et al. / Sensors and Actuators A 119 (2005) 398–404
reduce the risk of repetitive strain injuries (RSI) and noncontact ligament injuries. It is known that the conductivity and optical properties of conducting polymers such as polyaniline (PANi) and polypyrrole (PPy) depends on the oxidation state of the main chain as well as the degree of protonation of nitrogen atoms in the polymer backbone in PANi [13]. The physical properties of the coated fabrics depend on the nature of the monomer, the dopant, the substrate and the thickness of the coating employed. It is therefore possible to tailor parameters such as the conductivity and the hydrophobicity of the fabrics by choosing appropriate monomers and reaction conditions. However, existing C-P-based smart fabrics are typically thin pieces of conducting polymer coated textiles that work as two-dimensional strain gauges, i.e. they have to be stretched to give a change in conductivity and are not sensitive to force normal to the planar surface of the fabric. In this study we report the synthesis and properties of a new class of C-P-based smart fabric prepared by chemically coating polyurethane foam with conducting polymer (polypyrrole). These materials are soft, compressible and versatile and, in contrast to coated textiles, are sensitive to forces from all three dimensions. These foams are porous materials with large surface area to bulk ratios, have excellent fluid absorbance properties, and are robust to repeated compression and expansion. They could form the transducer basis for many sensing applications, both as physical transducers, and as chemical sensors, where the C-P coating properties are affected, for example, by certain gases or liquids.
2. Experimental 2.1. Chemicals and materials Pyrrole, naphthalene di-sulphonic acid (NDSA), and ferric chloride were obtained from Aldrich (Dublin, Ireland). Pyrrole was distilled prior to use. The NDSA, and ferric chloride were used without further purification. MilliQ water was used as the solvent for polymerisation and washing. The polyurethane (PU) foam substrate, was obtained from IRETEX (Leixlip, Ireland) and was first washed with soapy water and then rinsed with excess MilliQ water and dried in air prior to use. 2.2. Smart foam synthesis The conductive foams were prepared by in situ chemical polymerisation of the appropriate monomer, i.e. pyrrole, to retain the elasticity present in the foam substrate. The chemical polymerisation of pyrrole was modified from the method used by Oh et al. [11]. A 300 ml solution containing 0.04 M pyrrole and 5.4 mM NDSA was stirred in a 500 ml Duran borosilicate glass dish. A piece of polyurethane (10 cm × 10 cm × 1.7 cm), was soaked in the above solution for 2 h, ensuring that the foam was completely wetted with the
399
pyrrole/NDSA solution. Three hundred milliliters of 0.04 M FeCl3 was added and the mixture was stirred regularly for 2 h at room temperature and then allowed to stand overnight. The black PPy–PU foam was then removed from the container and triturated with MilliQ water to remove any loose bound PPy and blotted dry using tissue paper before being placed in an oven at 40 ◦ C overnight. The weight and the electrical conductivity (σ) of the PPy coated foam were then recorded. This coating procedure was repeated an additional three times to produce the final smart foam. 2.3. Conductance measurement 2.3.1. Sample preparation The PPy–PU foam was cut into specimens with dimensions of 1.7 cm × 1.7 cm × 1.3 cm. Conductive self-adhering foil (Radionics, Dublin, Ireland) was used to connect the two opposite end of the foam to the HP 34401A constant current multimeter (Hewlett Packard, Ireland). The data was collected by a PC using the software supplied by the manufacturer (HP multimeter software Version 1.1). 2.3.2. Experimental setup Three methods for determining the effect of pressure on the foam sample were used. The schematics of the setups used are given in Fig. 1A and B. The first setup (Fig. 1A) was made up of two PMMA platforms. Four metal pins (L = 10 cm, D = 0.2 cm) were fixed to each corner of a bottom platform (10.0 cm × 5.0 cm × 0.6 cm) so that there was 3.0 cm separation between the pins. Four holes (D = 0.21 cm) were drilled into the upper platform (5.5 cm × 6.0 cm × 0.2 cm, 6.203 g) so that it could thread through the four pins. The purpose was to give stability to the weight placed on the centre of the top platform. The sample prepared from above was put in between the two platforms and secured to the bottom platform by using two pieces of self-adhesive copper tape, separated by a distance of 1 cm. When known weights were placed onto the upper platform, the foam was compressed and the change in resistance was measured by the HP multimeter. The second setup, as shown in Fig. 1B, employed a clamp to sandwich the foam sample, again using self-adhesive copper tape as contact points between the sides of the foam and the multimeter. The clamp was gradually tightened to compress the foam sample from an initial length of Li to a final length of Lf . This produced a change in the resistance in the C-P coated foam that was measured by the multimeter. All experiments were run at room temperature and atmospheric pressure. The stress versus conductance profiles were obtained with an InstronTM machine (model 4202). The data was collected using the Series IX software as supplied by InstronTM , which recorded the load acting on the crosshead at a rate of 10 data pt/s while simultaneously monitoring the conductance and the length of the sample. The rate of loading and unloading was 2 mm/min throughout the experimentation. The
400
S. Brady et al. / Sensors and Actuators A 119 (2005) 398–404
Fig. 2. Structure of polypyrrole (a) and 1,5-naphthalene di-sulphonic disodium salt, Na2 NDSA (b).
Fig. 1. Schematic of the experimental setups, setup A is for compression study and setup B is for calibrating the foam sensor with known weight.
dimensions of the piston head were 50.44 mm × 32.86 mm. It was important that all the foam samples were cut to a smaller size than this to ensure that an even force was applied across to the entire foam area.
3. Results and discussion 3.1. General properties of PPy coated PU foam The PU foam used in this experiment was a nonconductive, light grey, soft, sponge like material that could be reversibly compressed. When it was coated with polypyrrole it became conductive and black in colour with a gold tint. The tactile properties (i.e. soft and compressible) were retained. The physical stability of the PPy coating on PU foam was found to be excellent. It was able to withstand vigorous washings (rubbing, squeezing) with water. When a piece of PU–PPy was cut through with a scalpel it was seen that the polypyrrole had completely penetrated into the PU matrix, resulting in a completely black mass inside out. The monomer pyrrole molecules were able to penetrate into the PU matrix and the oxidative polymerisation process resulted in a PPy–PU composite rather than simply a PPy coating adhered onto the PU surface. This finding explain the excellent
physical stability of the coating, however, it results in film thickness determination by methods such as SEM impractical because there is no distinctive layers to be seen. The structure of the coating material, i.e. the conducting form of polypyrrole and the dopant anion, naphthalene disulphonic acid is shown in Fig. 2. In general, highly doped polypyrrole contains 0.2–0.4 positive charges per monomer unit depending on the nature of the counter anion or dopant ion (represented here by X− ). These anions are incorporated into the polymer structure to maintain electroneutrality [14]. NDSA was used as a model dopant for PPy as it gives more stable polymer and yields highly conductive product in the order of mS/cm range. Other anions such as chloride, dodecylbenzene sulphonate, p-toluene sulphonate may be used in place of NDSA. A slow drift of baseline conductivity was observed for PPy coated PU foam when left in air therefore the foams were stored in a sealed polythene bag. This drift in baseline conductivity is due to the nucleophilic attack on the polypyrrole rather than due degradation of the polyurethane substrate. The crosslinking found in polyurethane foams makes it resilient to polymer degradation [15]. The nucleophilic attack on the polypyrrole layer gradually oxidises, by attacking the pyrrole ring to form carbonyl groups that break up the polymer backbone rendering it less conducting [16,17]. 3.2. Polypyrrole loading The sequential coating of PU foam with conducting polymers resulted in an increase of the overall weight of the foam and the conductivity of the foam also from being an insulating material to a conductive material (ca. 1.41 mS/cm). Table 1 clearly shows that the conductivity of the modified foam depends on the weight of conducting polymer deposited, which in turn depends on the number of coating layers deposited on to the foam substrate.
S. Brady et al. / Sensors and Actuators A 119 (2005) 398–404
401
Table 1 Effects of polymer loading on conductivity of PU foam, where wt and σ are the weight and resistance of the foam sample No. of coatings (n)
Total weight deposited (g)
Ratio (wtn /wt1 )*
Conductance, σ (mS/cm)
Ratio σ n /σ 1
0 1 2 3 4
0 0.1408 0.2815 0.7037 0.8394
– 1.00 1.99 4.99 5.96
– 0.000578 0.0714 1.25 1.41
– 1.00 123.50 2162.60 2439.40
The subscript n denotes the weight and resistance of the foam material after the nth polypyrrole deposition layer.
The first coating procedure resulted in a very thin polymer coating as most of the conducting polymer was in the form of a fine dispersion in the solvent rather than adhered to the foam. There was only a small gain in weight, with negligible conductance (5.78 × 10−4 mS/cm) being achieved. The second coating procedure added on a similar weight to the amount obtained from the first coating procedure, suggesting that the thickness of the PPy coating has roughly doubled (Table 1, wt2 /wt1 ≈ 2). However, the conductance measured was 7.14 × 10−2 mS/cm, which is two orders of magnitude higher than the single coated foam (Table 1, σ 2 /σ 1 ≈ 123). It is believed that the PPy film thickness (which relates to charge carrier density within the bulk polymer film) is critical for the coated foam to conduct electricity efficiently. After the third coating being put on, a ca. five times increase in PPy weight was obtained compared to the first coating, and resulted in an excellent electrical conductance of 1.25 mS/cm (Table 1). Further coating added produced much smaller PPy weight increases, and small improvements in conductivity. Therefore, we have taken conducting foams with coated with three layers of PPy as the optimised material to be used in all subsequent studies. It has been mentioned that the PPy coating penetrated into the PU matrix and it was not possible to determine PPy film thickness by SEM. A rough mathematical estimation was therefore used to calculate the film thickness from the weight deposited onto the foam based on an estimated surface area. However, because polyurethane foam is a porous material with irregular pore sizes, it is very difficult to even estimate the outer surface area of the foam let alone the overall surface area, therefore the data shown here is only for indicative purposes. The outer surface area of the foam, ignoring the pores and inner porous area, was 268 cm2 , one would expect the total surface area of the foam be much bigger (1–2 orders of magnitude bigger). The total weight of PPy put on after the first coating was 140 mg, and given that the density of PPy is close to 1, the volume of the coating would be approximately 0.14 cm3 . This crude calculation estimates that the first coating gave a film thickness of around 0.5–5 nm. The second coating added on approximately the same thickness to give an approximate 1–10 nm thickness range but with a big increase of conductance. In addition to the thickness increase, it is likely that the second coating lead to a much more homogenous coating of PPy throughout the surface of the foam resulting a big increase in conductivity. The third
coating gave an optimal conducting foam with estimated PPy thickness in the region of 2.5–25 nm. 3.3. Sensor responses The conductance of ICPs, in essence, depends on the density of the charge carrier, i.e. polarons, present in individual polymer chains and the efficiency of charge movement between chains. Hence, the conductivity of a continuous conducting polymer film is described by MacDiarmid [18] as σbulk = f (σintra ) + f (σinter ) + f (σdomain )
(1)
where σ bulk is the bulk conductivity of a conducting polymer, which is a cumulative result of the intra-molecular (σ intra ), inter-molecular (σ inter ), and the inter-domain (σ domain ) conductivities of the polymer film structure. One would expect the conductivity of the PPy coated PU foam also obey Eq. (1). Given that the PU foam contains a large number of empty holes, when a pressure is exerted to compress the foam, it reduces the foam’s overall volume, thus increasing the contacting area of the surface that is covered with PPy film. This in turn enhances the apparent density of the polaron and also shortens the conducting path length (increases magnitude of σ inter and σ domain ) to result in an increase in bulk conductivity. This effect was verified by using the setup shown in Fig. 1A, with which the change in resistance was determined when a foam sample was gradually compressed from an original length L0 to Li . The main feature in Fig. 3 shows two plots of dR (R0 − R) versus L0 /L generated from two repeats of the compression experiment. The use of dR instead of R was to change the plot from a negative slope to a positive slope and to normalise the data presentation. These two plots showed similar linear regions between L0 /L values 1–5 (R2 = 0.99, average slope is 16Ω for n = 2), beyond which the curves began to level off. This observation could be explained by the fact that there was a limit to how much a material can be compressed, when the conducting foam with a length L was compressed to reach Llimit then L0 /L became a constant and hence the resistance R reached a constant value. The maximum resistance change that could be obtained was denoted in Fig. 3 as dRlimit . In this case the value of dRlimit obtained for the conducting foam was approximately 82 k and the Llimit was roughly 1.9 mm from an original length of 1.7 cm (corresponding to ca. 89% reduction in length).
402
S. Brady et al. / Sensors and Actuators A 119 (2005) 398–404
Fig. 3. The resistance change (dR) vs. L0 /L plots obtained from two repeats of the foam compression study. The inset shows a plot of resistance change (dR) vs. % change in length using the same data set as the solid triangle plot shown in the main feature.
The inset in Fig. 3 is a plot of responses versus percentage change of length L. Two distinct regions could be seen where a rather small but steady initial decreases in resistance was observed, followed by much sharper changes after the foam was compressed to approximately 50% of its original length. This observation is easy to understand; initially when the PPy coated foam was compressed the interstitial spaces within the foam decreases gradually with very slow increases in the contacting area between PPy coated surfaces (i.e. air was being squeezed out). When the compression finally reached a critical point (ca. 50% of the original length) where a massive increase in the contact area was achieved and a sharp drop in resistance was observed. Further compression resulted in more intimate contact between PPy chains and a linear change (R2 = 0.99 between 50 and 90% length change) in resistance was observed. The responses of the conducting foam to various weight placed onto the foam (or force applied) was investigated using the setup as shown in Fig. 1B. The weight compressed the conducting foam to reduce the overall length of the material and resulted in an increase in conductivity. This phenomenon is similar to the compression experiment described above. When the conductance G (i.e. the inverse of the electrical resistance) of the conducting foam obtained was plotted against the force applied, linear plots (R2 = 0.98) were obtained for the two repeats shown in Fig. 4. The average sensitivity (n = 2) obtained were 0.0007 mS N−1 . The full dynamic range of response was not investigated because only limited weights could be fitted onto the small platform. The data indicates that the conductance G of the foam is proportional to the force applied onto it. Therefore a simple relationship can be established where G=
1 = kF + C R
(2)
where k is a proportionality constant and C a constant term.
Fig. 4. Plot of normalised conductance (G/G0 ) vs. the stress applied onto a conducting foam sample. Data was normalised by using the ration of the conductance (G) of the material to the baseline conductance of the material (G0 ).
The data shown in Figs. 3 and 4 indicate that the repeatability of the foam sensor was poor. The main reason for this is that when the foam has been subject to intense compression, it results in hysteresis in the PU substrate i.e. loss of internal energy, and it would take a long time (hours) to get back to normal baseline. This phenomenon can be seen in the data obtained with an InstronTM instrument where a foam sample was subject to a steady compression (2 mm/min) followed by a reverse unloading process during which the increase in stress and the change of the length of the foam was monitored. Fig. 5 shows the two consecutive loading/unloading cycles obtained that show the hysteresis effect when the foam sample was subject to an increasing load until the internal energy that maintained the structure of the foam was overcome. The first loading cycle showed an increase in strain with applied stress until a limit was attained. The reverse process showed that a shift of the strain was observed resulting from a loss of internal energy due to the foam had a slower recovery time than the releasing speed of the Instron crosshead. The second cycle resulted in smaller shift in the strain profiles and also that they were closer to the first unloading profile
Fig. 5. Plot of stress (force per unit area) vs. strain (L0 − L/L0 ) applied onto a conducting foam sample obtained by an InstronTM instrument. Upward arrows indicate loading and downward arrows indicate unloading process.
S. Brady et al. / Sensors and Actuators A 119 (2005) 398–404
403
Fig. 6. Normalised (R/R0 ) trace of PPy coated PU foam when repeatedly exposed to a force of 2.30 N.
suggesting that mechanically the foam was in a lower energy state. This unwanted effect may be minimised by limiting the amount of stress applied to the foam sensor because this hysteresis effect is much less significant if a small load (0.813 N/cm2 ) is applied for a short time which allows the PU substrate to recover quickly in ca. 5 s as shown in Fig. 6. Fig. 6 also shows the repeatability of the foam when subject to a small known applied weight. A fixed weight (235 g or 0.813 N/cm2 ) was repeatedly put onto and then taken off from a piece of conducting foam while continuously monitoring the conductivity. The data has been smoothed, using a 5-point moving average smoothing algorithm and normalised by dividing the resistance (R) by the baseline resistance (R0 ) of the sensor. The foam responded rapidly with a drop in resistance when the weight was placed on the upper platform and reached a pseudo-equilibrium within seconds and recovered in approximately 5 s after the load was removed. It can be seen from the trace that the baseline resistance is much noisier than the resistance compressed sample. It was because the foam, being a light-weight sponge like material, is sensitive to external vibration; when the foam was compressed it changes into a denser material and this vibration effect is less significant. Due to the mechanical properties of the PU foam different response characteristics of the PPy coated PU foam subject to different weights were observed. Fig. 7 shows the data obtained by monitoring the response of the conducting foam to three different applied weights that corresponding to values of 9.1, 13.7 and 27.5 N. The results indicate that the initial response to all weights used were very similar with the amplitude of the response dependent on the force applied to the sensor. However, the times to reach equilibrium were different and were found to depend on the weight used in such a manner that bigger applied weight resulted in shorter time to reach equilibrium. When the lightest weight of ca. 9.1 N was applied an interesting respond feature was observed where the weight seemed to have sunk down slowly with the foam structure adjusting to accommodate the weight. It resulted in a response time of ca. 50 s. When the weight applied was increased to13.7 N a rapid response time of ca. 15 s was obtained. The slow weight accommodation feature was barely
Fig. 7. Normalised (R0 − R) trace of PPy coated PU foam as pressure sensor when exposed to different forces.
visible suggested that a force or weight that exceeded the internal energy of the foam would yield a quick response. This hypothesis was proved by further increasing the weight to 27.5 N, which resulted in a rapid respond and it reached a plateau in ca. 10 s. The PPy coated PU foam has been used for developing a breathing monitor, whereby the foam sample is incorporated into a harness to wrap around the ribcage area. The movement of the ribcage during breathing exerts pressure on the conducting foam causing an increase in conductivity of the material. Fig. 8 shows a real time trace which illustrates the repeated movement of the ribcage of a volunteer (SB) during breathing. Initially a deep breathing pattern was observed from a standing posture that showed high amplitude and slow rate. When the subject changed to a sitting down posture and breathed normally a decrease in the amplitude of the signal with an increase in breathing rate was observed. When the subject resume the deep breathing exercise the amplitude of the responses increased and the rate slowed down. This simple experiment shows that the breathing device is able to trace the rate and amplitude of breathing. Therefore it may be useful, with optimised sensor design that incorporated into wearable garment, to monitor breathing of infants or other patients that required special care. The potential of this conducting foam to be used in real life application has been demonstrated. However, there are
Fig. 8. Real time trace of PPy coated PU foam as pressure sensor to monitor ribcage movement while breathing.
404
S. Brady et al. / Sensors and Actuators A 119 (2005) 398–404
limitations concerning both the conducting polymers and the substrate material that are under investigation and these may seriously restrict the practicality of these material. The most important limitations are: (1) drift in conductivity of the conducting foam over time and (2) foam hysteresis after compression. The first disadvantage (1) is completely dependent on the chemical and physical stability of the conducting polymer coating. The cause of drift can be due to environmental interferences such as humidity effects and de-doping of PPy by amines that are present in the atmosphere. Such effects may be prevented if the foam is sealed in an airtight environment. The mechanical properties of the PU substrate are largely responsible for (2). Therefore further research is needed to find conducting polymers that are very stable and offer a compatible surface for ICPs. The identification of substrate foam materials that are robust, and free from hysteresis when repeatedly compressed is also important for the sensor to give reliable results.
[6] P. Audebret, N. Girult, T. Kaneko, Synth. Met. 53 (1993) 251. [7] J.Y. Lee, D.Y. Kim, C.Y. Kim, Synth. Met. 74 (1995) 103. [8] P.J. Kulesza, Proceedings of the Third Baltic Electrochemical Conference, 2003. [9] J.A. Paradiso, K. Hsiao, A.Y. Benbasat, Z. Teegarden, IBM Syst. J. 39 (2000) 511. [10] Sensor Products Inc. See http://www.sensorprod.com/podiascan.html (accessed 27 February 2004). [11] K.W. Oh, H.J. Park, S.H. Kim, J. Appl. Polym. Sci. 88 (2003) 1225. [12] P. Waters, Textile News Online 6, 2002. See http://www.tft.csiro.au/textile news/2002 1q/knowing cloths.html (accessed 27 February 2004). [13] R.S. Kohlman, A.J. Epstein, in: T.R. Skotheim, R.L. Elsenbaumer, J.R. Reynolds (Eds.), Handbook of Conducting Polymers, Marcel Dekker, New York, 1997, p. 85. [14] D. Kincal, A. Kumar, A.D. Child, J.R. Reynolds, Synth. Met. 92 (1998) 53. [15] M. Rutkowska, K. Krasowska, A. Heimowska, I. Steinka, H. Janik, Polym. Degrad. Stabil. 76 (2002) 233. [16] R. Mazeikiene, A. Malinauskas, Polym. Degrad. Stabil. 75 (2002) 255. [17] M.M. Chehimi, E. Abdeljalil, Synth. Met., in press. [18] A.G. MacDiarmid, Polyaniline and polypyrrole: where are we headed? Synth. Met. 84 (1997) 27.
Biographies 4. Conclusion A simple novel pressure sensing material has been developed and calibrated that combines the compressible characteristic of polyurethane foam and the inherent conducting ability of polypyrrole. The conductance of the conducting foam is found to change linearly with the force applied. The application of this conducting foam in wearable sensor has been demonstrated.
Acknowledgements We would like to acknowledge an IRCSET grant for SB RS/2002/765-1 and support from Science Foundation Ireland (Adaptive Information Cluster Award).
References [1] G.G. Wallace, M. Smyth, H. Zhoa, TRAC 18 (1999) 245. [2] M. Vidotti, L.H. Dall’Antonia, S.I. Cordoba de Torresi, K. Bergamaski, F.C. Nart, Anal. Chim. Acta 489 (2003) 207. [3] J. Wu, X. Yu, H. Lord, J. Pawliszyn, Analyst 125 (2000) 391. [4] J.D. Madden, R.A. Cush, T.S. Kanigan, I.W. Hunter, Synth. Met. 113 (2000) 185. [5] M.A. El-Sherif, J. Yuan, A. MacDiarmid, 11 (2000) 407.
Sarah Brady obtained her BSc degree in 2002 in Analytical Science from Dublin City University. She is currently a PhD student in the School of Chemical Sciences at the same University. Her research project involves the development of sensor/actuator systems based on inherently conducting polymer. This smart foam project pioneers the use of conducting foam for pressure sensing and as smart filter for chemical sensing. Dermot Diamond obtained his BSc, MSc, PhD and DSc from Queen’s University, Belfast. He is currently leader of the Adaptive Sensors Group at Dublin City University, Ireland, and director of the Adaptive Information Cluster, a research initiative funded by Science Foundation Ireland. His research interests include molecular recognition; host–guest chemistry; ligand design and synthesis; electrochemical and optical chemical sensors and biosensors; lab on a chip devices; with applications in environmental, clinical, food quality, and process monitoring. More recently, he has focused on the development and applications of fully autonomous sensing devices; wireless sensors; sensor networking; and developing the potential of devices and sensors to provide information for wireless networked systems. K.T. Lau was a graduate of Royal Holloway and Bedford New College (now the Royal Holloway College), University of London. He then acquired a MSc degree in Chemical Research on array sensing with Quartz Crystal Microbalance from Birkbeck College, University of London, where he stayed on to obtained a PhD on the development of amperometric biosensor for glucose measurement. He is currently a senior research fellow in the National Centre for Sensor Research, Dublin City University. His main research area is in sensor development and molecular recognition.