electrospn 21 macagnano-full

The International Istanbul Textile Congress 2013 May 30th to June 1th 2013, Istanbul, Turkey

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ELECTROSPUN SMART FABRICS TOWARDS WEARABLE SENSORS

E. ZAMPETTI1, A. BEARZOTTI1, F. DE CESARE2,1 and A. MACAGNANO1 1Institute for Microelectronics and Microsystems-National Research Council, Rome ITALY

2DIBAF-University of Tuscia, Viterbo ITALY [email protected]

Abstract: Electrospun fabrics of PANi-PHB blends were manufactured and studied as potentials sensors for gas and volatile organic compounds. The resulting fibrous scaffolds were highly porous, with interconnected void volumes, high surface-to-volume ratios and dimensionally homogeneous fibres. A proper concentration of PANi allowed electrical conductivity ((R~1.6·106 Ohm) through the fabrics at low applied potential (1V) and sensitivity to acetone vapours in different humidity conditions. Keywords: electrospinning, PANi-PHB blends, nanofibres, wearable biodegradable sensors 1. Introduction Clothing is the interface between human body and environment. There is a need for an “ambient intelligence” in which smart devices are integrated into the everyday surroundings to provide diverse services to everyone. As our lives become more complex, people want “ambient intelligence” to be personalized, embedded and usable anytime and anywhere. Therefore clothing would be an ideal place for smart systems, namely capable of sensing environment and communicating with the wearer. While constant efforts have been made toward miniaturization of electronic components for wearable electronics, true “smart clothing” requires full textile materials for all components [1]. At the same time, wearable smart systems have to be embedded in textiles so that both electronic functionality and textile characteristics are retained. Smart clothing should be easy to maintain and use, and washable like ordinary textiles. Therefore, combining wearable technology and clothing textile science is a key factor to achieve smart clothing for actual wearability. Recently a great number of studies reported noteworthy results about electrospun nanofibrous based sensors. Such nanostructured materials, mainly due to the enhancement of their surface area to volume ratios, achieved two additional and essential features for sensors: high sensitivity and fast response time. Electrospinning technique has been confirmed to be a relatively simple and cost-effective technology capable of producing fabrics with a huge increasing of sorption sites within a single step [2]. Slight modifications of the starting solution are sufficient to change remarkably the fiber-tissue structure and then the sensing properties. The ability to manufacture a wide variety of fabrics (eg. conformable, biocompatible, water-resistant, etc.) enclosing interacting materials to gases and volatile organic compounds (VOCs), gives the electrospinning technique the opportunity to provide potential wearable sensor systems for diagnostic (physiological and biochemical parameters) as well as monitoring applications (toxics in environment). The combination of electrospun sensors and printable microelectronics, will make available recording, analysing, storing, sending and displaying data, which is a new dimension of not-expensive smart systems. They should extend the users’ senses providing useful information anytime and anywhere the users go. Conductivity in textiles is essential to smart clothing since electrical conductivity provides pathways to carry information or energy for various functions. Conductivity in textiles can be imparted at various textile stages. Previously, suitable electrospun conductive polymers (CPs) have been specifically designed and investigated for developing potentially wearable smart sensors whose electrical properties change upon interactions with the analytes [3]. Among CPs, polyaniline (PANi) is a polymer which has been frequently investigated as gas sensing material [4]. Indeed, additionally to its environmental stability (up to 300 °C), the doping process is directly related to the sensing mechanism. For instance, a dedoping effect through deprotonation is thought to happen when ammonia gas is adsorbed onto PANi molecules. On the contrary, acidic gases (HCl, H2S, and CO2 with water) can cause to dope it through protonation. PANi solvents (VOCs) too change the conductivity of the solid state polymer. Each of these changes, following the polymer exposure to analytes, can be detected as current variation commonly measured by suitable resistors. The most common chemoresistor consists of one or several pairs of electrodes (interdigitated electrodes—IDEs), fabricated through cheap and convenient processes. Preliminary results about sensing properties of some fabrics made of blends of PANi and poly-3-hydroxybutyrate (PHB), a biodegradable polymer, have been here reported [5]. Such a polymer is water insoluble and relatively resistant to hydrolytic degradation: these features differentiate PHB from most other currently available biodegradable plastics, which are either water


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soluble or moisture sensitive. In addition, its good oxygen permeabilty, UV resistance, tensile strength close to that of polypropylene (40 Mpa), biocompatibility and non toxicity make it a potentially good material for clothing in the future. The resulting fabric should be non-toxic and capable of maintaining good mechanical integrity until degraded. This study should create a dowell of a database for the production of fabrics such as biomedical devices, such as bandages, patches, and masks able to detect and/or monitor certain physiological parameters (such as heart rate, respiration frequency, some volatile bio-markers, temperature, etc..) or even wound infection and /or injury from pathogens. Therefore the sensing properties of such nanofibres have been investigate in order to understand if they could be used for sensing gas and volatile organic compounds (such as acetone, biomarker of several human diseases), overall for biomedical tools. 2. Materials and methods 2.1 Polymer Blends Preparation Polyaniline emeraldine base (PANi, MW=10,000), 10-camphorsulfonic acid (CSA), (R)-3-Hydroxybutyric acid polymerized (PHB) and 2,2,2-Trifluoroethanol (TFE) were purchased from Aldrich and used without further purification. Polyaniline doping was carried out stirring for a week 300mg of PANi with 200 mg di CSA in 20 ml di TFE. PHB (110 mg) was solved in 1 g of solvent and vortex stirred for fifteen days until obtaining a white and seemengly homogeneous solution. Three different blends were prepared adding increasing amounts of the deep-green solution of PANi to the solution of PHB (A=1:10 v/v, B=2:10 v/v, C=3:10 v/v of PANi/PHB, respectively) and left under stirring for a period of 48 hours. Two layers (named with letter and numbered letter, respectively) for each concentration were preliminarily investigated and tested as sensors. 2.2 Interdigitated sensors The base transducer elements [6] used in the present work were interdigitated electrodes with 40 pairs of electrodes and gaps of 20 µm. Each electrode (5620 µm long) was fabricated on a 4 inches oxidized silicon wafer, by means of a standard lift-off photolithographic process. The metallization layer was made by evaporation of a chromium-gold film (150 nm thick). 2.3 Electrospinning setup and deposition The depositions were carried out in a homemade clean box comprised of temperature and humidity sensors (24°C, 35% RH). The syringe containing the polymer blend was placed on the syringe pump (KDS-200, KD Scientific, feed rate 600 µl/h) with the tip-collector distance of about 15 cm. The collector was a rotating conductive cylinder (500 rpm, Ø=45mm), housing the transducers, able to increase the quantity of fibres assembled per unit of time. The steel needle with flat tip (Øin=300 µm) was connected to the high voltage generator (ranged between 5~10 kV DC) and the deposition time was fixed at 600 s for all the polymer materials. After each deposition, the transducers were housed in an incubator at 40 °C for 24 hours, in synthetic air flow, in order to accomplish the solvent evaporation. 2.4 Characterization Morphology and structure of the electrospun fabrics deposited on the transducers were investigated by optical microscopy (Olympus MX50 Optical microscope) and Atomic Force Microscopy (AFM, PSIA XE100). The chemoresistors were placed in a suitable measurement chamber (≈1 ml volume) constructed from PTFE and connected to an electrometer (Keithley 6517 Electrometer) capable of measuring the current flowing through each IDE when a fixed potential was applied to it and of sending data to a PC. Dynamic measurements were performed at room temperature using both a 4-channel MKS 247 managing four MKS mass flow controllers (MFC) and an Environics S4000 (Environics Inc.) flow controller managed with its own software. Both synthetic dry air(Praxair-SIAD 5.0) and humidified air, by water vapours at known percentages, were used as carrier gases for measuring the analyte. Acetone (CHROMASOLV® Plus, for HPLC, ≥99.9%, Sigma-Aldrich) was placed into a bubbler in order to supply homogeneous and known vapour concentrations to the sensors. 3. Results IDEs coatings, after 10 min of electrospun deposition, were, at first, investigated by capturing images using optical microscopy (Figure 1). A, B and C fabrics (as well as A1, B1 and C1) were highly porous, showing interconnected void volumes and high surface-to-volume ratios. Furthermore, the polymer fibres covered all the electrodes of each IDE and the gaps between them, presumably providing the electrical connection


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within each transducer. Such resulting fabrics, proportionally to the amount of PANI, had three shades of green: the most intense green was related to the highest PANi concentration.

Figure 1. Layout of an electrospinning setup comprising a syringe pump and a rotating grounded cylinder housing an interdigitated electrode to be coated with a PANi-PHB fibrous layer.

AFM micrographs provided information about the morphology of the single fibres: they appeared smooth and very thin in diameter when PANI content was low but gradually rough and diameter increased (up to 160 nm) when the CP increased. In Figure 2, a 3D AFM image (on the right) of a C fibre on silicon substrate is depicted. Specifically, the resulting fibre appeared rough with some lumps probably due to increased concentration of polyaniline. Fibres appeared perfectly laid on the silicon substrate. Its surface properties were enhanced by the AFM error image (on the left), capable of better depicting the features correlated to the structures of the edges.

Figure 2. AFM micrographs of a single PANi-PHB fibre of C fabric: on the left an error signal image, on the right a 3D view

The electrical parameters of the PANi-PHB coated IDEs were tested by measuring the current changes under varying voltage. Figure 3 depicts a comparison between two I-V curves of the sensors when dried or humidified synthetic air was flowed throughout the measurement chamber. All the measurements were performed at room temperature. The resulting nearly linear shape (Ohmic behaviour) within the selected voltage range (-2 V to +2 V) indicated constant resistance values for all of the nanostructured films. More in detail, the measured resistance values were related to the polyaniline concentration along fibres (A≈49·106 Ohm, B≈12·106 Ohm, C≈1.6·106 Ohm). The lowest level of resistance in the C sensor prevented high values of thermal electronic noise and provided the possibility of working at very low voltages. Additionally, when measurements were carried out in air at 38% RH, only the sensor containing the higher amount of polyaniline (C) was slightly affected, maintaining comparable resistance values up to 50%RH (Figure 4, on the left). Instead A and B sensors were highly sensitive to humidity, changing significantly the current in


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response to variations of water vapours percentages (increasing in current when relative humidity increased). However I-V curve shapes didn’t change when the environmental humidity varied.

Figure 3. Current-Voltage curves of the PANi-PHB fabrics with different concentrations of PANi within fibres under dried and humidified environmental conditions

Figure 4 (on the right) shows the normalized transient responses of both sensors C and C1 to 1% of the headspace of acetone under synthetic air. Current decreased when acetone interacted with the fibrous layer. Both the measurements were comparable, suggesting a possible reproducibility of the coating features with electrospinning deposition. Measures were carried out for 15 min providing a limit of revelation of about 90 ppm of acetone. However long measurements determined hysteresis of the sensor (i.e. a partial recovery), thus 2 min impulses of acetone vapours were performed (Figure 5, on the left) both under synthetic and humidified air. This reduced exposure time to the VOC allowed restoring the sensor current values as before measurement.

Figure 4. Current-Voltage curves of C fibrous layer under dry and humidified air (on the left); normalized responses of C and C1 sensors to 0.01 p/p0 acetone (15 min of exposure) in dry air (on the right)

The similar slope of C response curves despite of the different current values (Figure 5, on the right) both in synthetic and humidified air suggests comparable sensitivity values to the analyte not affected by environment humidity.


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Figure 5. Sensor responses of C and C1 to 2 min of exposure to increasing concentrations of acetone (on the left) at 38% RH; response curves of C in dry (red) and humidified (black) environment to increasing partial pressure values of

acetone

4. Conclusions Electrospun fabrics of PANi-PHB have been manufactured and studied as potentials sensors for gas and VOCs. The fibrous scaffold appeared highly porous, showing interconnected void volumes and high surface-to-volume ratios with dimensionally homogeneous fibres. AFM micrographs described rough fibres with some small lumps when higher concentrations of polyaniline were blended. Some preliminary results about PANi-PHB fabrics as potential sensing textile showed that PANi amount is a key parameter both for the conductivity (low noise), the influence of environmental parameters (humidity) and finally interaction with acetone vapours, a known biomarker related to several human metabolic diseases. The intriguing features of PHB such as biocompatibility and non-toxicity as well as biodegradability and insolubility in water together with the sensing features of PANi actually seem to confirm the expected potentials of these structures as smart materials towards disposable and low cost wearable sensors. Further studies will obviously be necessary to study both the mechanical, physical and chemical properties of the fabrics produced when exposed to extreme environments, as well as to obtain better performances when interacting with the analytes (i.e. a higher sensitivity, selectivity, etc.). References [1] Cho G.: Smart Clothing Technology and Applications, CRC PressTaylor & Francis Group, ISBN 978-1-

4200-8852-6, NY USA, (2010) [2] Ding B., Wang M., Wang X., Yu J. and Sun G.: Electrospun nanomaterials for ultrasensitive sensors,

Materials Today, Vol. 13 (2010) No.11, pp. 16-27, ISSN 1369-7021 [3] Zampetti E., Pantalei S., Scalese S., Bearzotti A., De Cesare F., Spinella C., Macagnano A., Biomimetic

sensing layer based on electrospun conductive polymer webs, Biosensors and Bioelectronics, Vol. 26 (2011), pp. 2460-2465, ISSN: 0956-5663

[4] Macagnano A., Zampetti E., Pantalei S., De Cesare F., Bearzotti A., Persaud K.C., Nanofibrous PANI-based conductive polymers for trace gas analysis, Thin Solid Films, Vol. 520, No. 3 (2011), pp. 978-985, ISSN:0040-6090

[5] Jin L., Wang T., Zhu M-L., Leach M.K., Naim Y.I., Corey J.M., Feng Z-Q., Jiang Q., Electrospun Fibers and Tissue Engineering, Journal of Biomedical Nanotechnology, Vol. 8, No. 1 (2012), pp. 1-9

[6] Janata, J., Josowicz, M., Conducting polymers in electronic chemical sensors, Nature Materials, Vol. 2 (2003), pp. 19-24.

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