A stretchable humidity sensor based on a wrinkled polyaniline nanostructure

Abstract

A stretchable humidity sensor which has mechanical stability has remained a challenging issue in stretchable electronics. Here, we report a novel stretchable nanostructured polyaniline humidity sensor. The sensor was fabricated by simple fabrication methods such as a pre-stretching process and self-assembly texturing process. The periodically wrinkled structure supported by microstructured elastomeric substrates provides an excellent stretchability. The micro-textured substrate was introduced for solving the crack problem by the Poisson effect as a stretchable sensor. The sensor maintains its humidity sensitivity well at different elongations. To the best of our knowledge, this is the first report of a stretchable humidity sensor.

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Stretchable electronics is a challenging field and so is the development of sensory skins for robotics, structural health monitors, and wearable communication devices, beyond flexible electronics.¹ Recently, there have been numerous stretchable electronic devices that use components such as elastic conductors,^2,3 light emission diodes,^4,5 field effect transistors⁶ and temperature sensors.⁷ One of the most critical issues in stretchable sensors is realizing wearable and electronic skins, keeping their sensitivity reliable level.

Humidity sensors have gained great attention for their practical applications in industrial fields, laboratory environment, and our daily life.⁸ Flexible humidity sensors especially attract attention where moisture sensing under deformation is required. Recently, deformable humidity sensors have been developed that use only flexible devices.^9–12 However, the operation of such devices was feasible for only small deformations as there would be a rupture of the sensing material at high deformations.

Here, we report a highly stretchable nanostructured polyaniline humidity sensor. The sensor maintains its humidity sensitivity stably at the each differed elongations. To our best knowledge, this is the first report for a stretchable humidity sensor.

The stretchable sensor consisted of polydimethylsiloxane (PDMS) as an elastic substrate and polyaniline (PANI) layer as a sensing material as shown in the Fig. 1. PANI which is a kind of conducting polymers has an excellent electric property and humidity sensitivity.^13–15 For better sensing performance, the PANI was synthesized on the PDMS as a form of hairy nanostructure (diameter: 20–40 nm; length: ≈100 nm, Fig. 2). First, flat PDMS substrates and microtextured PDMS substrates were prepared by silicon templates fabricated by typical microelectromechanical system (MEMS) process. PDMS solution was prepared by mixing the base and agent (Dow-Corning, USA) of ratio 100 : 10 (w/w). The solution poured to the template and cured at 65 °C for 3 hours in a convection oven.

Fig. 1 Fabrication of a stretchable nanostructured polyaniline humidity sensor. (a and b) Preparation of pre-strained elastic substrate. (c) PANI nanostructures coating by a dilute polymerization. (d) Wrinkled PANI nanostructured surface after removing of the pre-strain.

Fig. 2 (a and b) The SEM images of as-prepared PANI nanostructured surface. (c and d) The PANI layer was wrinkled in the vertical direction to the stretching direction (yellow allow) and cracked the horizontal direction due to the Poisson effect. (e) The SEM image of PANI structure on a micro grooved substrate and. (f) Magnified image on the side wall of the micro groove. (g) Wrinkled PANI structures after releasing. Releasing direction corresponds to the yellow allow. (h) Magnified image. The margin PANI layer on the side wall prevented the crack formation due to the Poisson effect.

In general, the PANI has higher modulus (≈1 GPa (ref. 16)) than that of PDMS (<1 MPa (ref. 17)). Due to the Young's modulus mismatch, the generation of stress at the interface of the elastic and non-elastic caused the surface to fracture in high strain conditions as shown in our previous work.¹⁸ Thus, to prevent and suppress fracturing of sensing surface in high deformation, pre-stretched elastic substrates PDMS were prepared as shown in the Fig. 1. The PDMS substrate was pre-stretched strain of 50% and the stretched PDMS were coated with PANI hairy nanostructures using a dilute polymerization as following.^18–20

The PDMS membrane was immersed in an aqueous solution containing 1 M HClO₄ (Samchun Pure Chemical Co., Korea), 6.7 mM ammonium persulfate (APS, Sigma Aldrich Co., USA), and 10 mM aniline monomer (Sigma Aldrich Co., USA). The aniline monomers were polymerized at a temperature of 0–4 °C, with 12 h of agitation (shaking of the mixture). After synthesis, the membranes were rinsed in a deionized (DI) water flow to remove the remaining PANI residue. The membranes were then dried using N₂ gas and placed in a desiccator for 1 day. Finally, a wrinkled stretchable sensor was obtained after releasing (Fig. 1(d)). This winkled PANI sensor could stretch reversibly as the wrinkles suppress the occurring of PANI fracture in the stretching direction.

The schematic of experimental setting is showed in the Fig. S1 (see ESI†). The direct current of a sensor was measured as a relative humidity (RH) using semiconductor analyser (HP 4156A, USA). All experiments were conducted at room temperature under the voltage of DC 1 V. The humidity environments were controlled using supersaturation aqueous solutions of different salts. LiCl, MgCl₂, Mg(NO₃)₂, NaCl, KCl, and KNO₃ in a closed small chamber yielded 11, 33, 54, 75, 85 and 95% RH, respectively^21,22 (see Table S1 in ESI†).

Fig. 2(a–d) show SEM images of as-prepared PANI nanostructured surface and released surface, respectively. During the stretching cycle, no crack was observed in stretching direction (direction of yellow arrows) and the PANI structures maintained their morphology, but some crack formed its horizontal direction due to the Poisson effect, deforming with the negative ratio of transverse to axial strain. If the applied force was removed, then the PANI layer tended to revert back to its original shape. However, the horizontal cracks could affect the sensing performance of the sensor because the discrete morphology could interrupt the flow of electron or the conductivity of crack could be changed according to the deformation in the PANI sensor. Thus, we propose a novel micro grooved structure that prevents crack by the Poisson effect. Fig. 2(e and f) present the SEM images of a PANI layer on a micro grooved PDMS substrate (width of 20 μm, height of ca. 1 μm). When the pre-stretched substrate released, the layer was wrinkled and shrunk in horizontal direction. Also, by the Poisson effect, the layer extended in the vertical direction. Unlike the flat substrate, the textured substrate hardly formed cracks because of marginal space on the side wall of the micro groove (Fig. 2(g and h)). The delaminated layer at the side wall maintained its morphology through the cyclic extension test. The marginal space is also not crucial to sensitivity because the sensor was operated by DC measurement but it can affect the performance at AC measurement. More detail analysis discussed in ESI.† Consequently, the pre-strained process and the micro structured surface allow maintaining stably morphology of PANI layer without fracturing under harsh dynamic deforming conditions such as stretching.

The response of the humidity sensor with wrinkled PANI nanostructured surface was measured at various relative humidity (RH) environments. As shown in the Fig. 3(a), the PANI sensor responded linearly to humidity and sustained a stable state at each RH when the sensor was stretched at the elongation of 40%. Similar responses were also observed at different ranges of elongation.

Fig. 3 (a) The dependence of response on the RH for the wrinkled PANI nanostructured humidity sensor at 40% elongation. (b) The dependence of response on the various elongations.

After the doping process of PANI, it has a chain with the protonated reduced and oxidized form. If water interacts with C=C, the conjugated double bonds of the PANI chain, it can easily release proton. Because of these protons, the transferring of the electron from the protonated reduced form to the protonated oxidized form could be easier. Thus, the amount of absorbed water molecules plays a important role in the change of the conductivity depending on each RH.⁸ The inset of Fig. 3(a) shows linear dependence on the increasing humidity of the sensor from 11% to 95% and the response and the recovery properties of the stretchable humidity sensor were also obtained (see Fig. S3 in ESI†).

The response behavior of the stretched humidity sensor was also obtained at various elongations (10–40%), as shown in Fig. 3(b). We have tested the sensitivity of humidity up to 40% strain. Theoretically, the sensor can also operate at 50% strain because the pre-strain was 50%. During the drying process, however, the PANI layer was somewhat contracted compared to as-synthesized PANI structure as water evaporated, resulting in cracking at 50% strain.

Under the various fixed RHs (11, 33, 54, 75, 85 and 95% RH), the sensor maintained stably its humidity sensitivity at the differed elongation. This mechanically stable response is because of our unique geometries – the wrinkled and micro textured surface. The geometry helped to release the interfacial stress that occurred during the high deformation.

The measured current of the sensor was slightly decreased, especially at low level humidity and high elongation range above 30%. It is not caused by structural rupture of the PANI layer during experiments. An electrical path of the sensor between electrodes can be longer because the attached PANI surface with elastomeric substrate have not completely returned to its original length. It had influence on the resistance of the humidity sensor to increase slightly. The reason strain had an effect on humidity sensing results is also because of the difference of water ratio with respect to the sensing area. At high humidity, sufficient amount of water molecules cover the PANI surface in all elongations and it could form a dominant current pathway. However, the current of the sensor decreased as the sensor was stretched at low humidity because there were relatively insufficient amount of water molecules with respect to the sensing area.

Although we have studied the properties of the humidity sensor with DC bias measuring method, the DC measuring method could polarize the sensing material.^23,24 Therefore, when the subsequent experiments were conducted continuously, there can be a signal drift by the influence of the continuous experiments.

In summary, we demonstrated the novel stretchable nanostructured polyaniline humidity sensor which was fabricated by simple fabrication methods such as pre-stretching and a diluted polymerization. The stretchability was realized by the periodically wrinkled structure supported by elastomeric substrates. The micro-textured structure on the substrate was suggested for cracking problem by Poisson effect as a stretchable sensor. The fabricated sensors stably maintained their performance up to strain of 40%. We believe that the advantages of the method described here can be extended to sensory skins for robotics, structural health monitors and other many applications of the stretchable electronics.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (no. 2012R1A2A2A06047424).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details and data of fabricated the nanostructures. See DOI: 10.1039/c4ra04938a

‡ These authors contributed equally to this work.

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