Flexible Thick-Film Electrochemical Sensors: Impact of Mechanical Bending and Stress on the Electrochemical Behavior

Abstract

The influence of the mechanical bending, rolling and crimping of flexible screen-printed electrodes upon their electrical properties and electrochemical behavior has been elucidated. Three different flexible plastic substrates, Mylar, polyethylene naphthalate (PEN), and Kapton, have been tested in connection to the printing of graphite ink working electrodes. Our data indicate that flexible printed electrodes can be bent to extremely small radii of curvature and still function well, despite a marginal increase the electrical resistance. Below critical radii of curvature of ~8 mm, full recovery of the electrical resistance occurs upon strain release. The electrochemical response is maintained for sub-mm bending radii and a 180° pinch of the electrode does not lead to device failure. The electrodes appear to be resistant to repeated bending. Such capabilities are demonstrated using model compounds, including ferrocyanide, trinitrotoluene (TNT) and nitronaphthalene (NN). These printed electrodes hold great promise for widespread applications requiring flexible, yet robust non-planar sensing devices.

Figure 1

Schematic of screen-printed electrode (SPE) (A) on a flexible substrate (a) with a thick-film carbon working electrode (b). (B) Schematic of the 90° longitudinal inward bend of the flexible SPE. (C) Photo of the 180° longitudinal inward bend of a SPE on a Mylar substrate.

Figure 2

Optical images of strain test of printed electrodes on different flexible plastic substrates: Kapton (A), PEN (B), and Mylar (C) before (a) and after (b) bending, along with a x100 zoom after bending (c). The arrow in images (b) indicates the place where bending took place.

Figure 3

Impact of controlled bending of SPE cured for 3 min (●), 10 min (▲) and 60 min (■) on the resistivity during (A) and after (B) bending. The dashed lines represent the resistivity of the unstressed electrodes. Solid lines are provided as a guide for the reader’s eyes.

Figure 4

Background cyclic voltammograms of a 0.1M PBS (pH 6.5) solution recorded at a flexible PEN substrate SPE before (A) and after (B–D) inward longitudinal bending at different angles: 90° (B), 180° (C), and 180° pinched (D). Bending time, 5 min; scan rate, 0.1 V/s.

Figure 5

Cyclic voltammograms of 5mM potassium ferrocyanide recorded (A): using SPEs based on alumina (a) and flexible PEN (b) substrates; (B): using flexible PEN substrate SPE before (b) and after inward longitudinal bending at different angles: 90° (c), 180° (d), and 180° pinched (e). Bending time, 5 min; scan rate, 0.1 V/s; electrolyte, 0.8M KCl.

Figure 6

SW voltammograms of 50 ppm TNT in 5 % of ACN/0.1 M PBS (pH 6.5) recorded at alumina (A, C), PEN (A, B) and Kapton (C, D) SPEs. Background voltammograms at alumina (A,C; a), PEN (A,B; b) and Kapton (C,D;b) based SPEs; TNT response at the alumina (A,C; d), flexible PEN (A,B; d) and flexible Kapton (C,D; d), including TNT signals at different bending angles of the flexibles SPE (B,D): 90° (d₂), 180° (d₃) and 180° pinched (d₄). SWV parameters: amplitude, 25 mV; potential step, 4 mV and frequency of 30 Hz.

Figure 7

Stability of the SWV response of 1-NNP (20 ppm in 1M NaCl) measured at a PEN flexible SPE before and after repeated 5 min 180° bending. The average signal stability is counted from the heights of the peaks and represents measurements at 5 electrodes where 100% corresponds to the unstressed electrode (A). The SW voltammograms of the supporting electrolyte and of 20 ppm 1-NNP before and after bend are shown in Fig. 7B (a and b, respectively). SWV parameters, as in Figure 6.