Experimental Characterization of Inkjet-Printed Stretchable Circuits for Wearable Sensor Applications

Abstract

This paper introduces a cost-effective method for the fabrication of stretchable circuits on polydimethylsiloxane (PDMS) using inkjet printing of silver nanoparticle ink. The fabrication method, presented here, allows for the development of fully stretchable and wearable sensors. Inkjet-printed sinusoidal and horseshoe patterns are experimentally characterized in terms of the effect of their geometry on stretchability, while maintaining adequate electrical conductivity. The optimal fabricated circuit, with a horseshoe pattern at an angle of 45°, is capable of undergoing an axial stretch up to a strain of 25% with a resistance under 800 Ω. The conductivity of the circuit is fully reversible once it is returned to its pre-stretching state. The circuit could also undergo up to 3000 stretching cycles without exhibiting a significant change in its conductivity. In addition, the successful development of a novel inkjet-printed fully stretchable and wearable version of the conventional pulse oximeter is demonstrated. Finally, the resulting sensor is evaluated in comparison to its commercially available counterpart.

Keywords: inkjet printing; printed sensors; silver nanoparticles; stretchable sensors.

Figure A1

Average resistances of two sinusoidal samples at a width of 1 mm and various amplitudes as the applied strain increases (loading) and decreases (unloading). Error bars indicate the standard deviation in the values of the resistances.

Figure A2

Average resistances of two sinusoidal samples at a width of 1.2 mm and various amplitudes as the applied strain increases (loading) and decreases (unloading). Error bars indicate the standard deviation in the values of the resistances.

Figure A3

Average resistances of two sinusoidal samples at a width of 1.5 mm and various amplitudes as the applied strain increases (loading) and decreases (unloading). Error bars indicate the standard deviation in the values of the resistances.

Figure A4

Average resistances of two sinusoidal samples at various cycles as the applied strain increases (loading) and decreases (unloading). Error bars indicate the standard deviation in the values of the resistances.

Figure A5

Average resistances per unit length (Ω/mm) of two horseshoe samples at various angles as the applied strain increases (loading) and decreases (unloading). Error bars indicate the standard deviation in the values of the resistances.

Figure 1

Stretching tool used to apply axial strain while measuring the electrical resistance of the printed lines.

Figure 2

Sinusoidal patterns (not to scale) at various amplitudes, line widths, and cycles created using GIMP software.

Figure 3

Horseshoe patterns at various angles using AutoCAD software.

Figure 4

Optical microscope images of printed lines with DS of 5, 10, 15, 20, 25, and 30 µm.

Figure 5

Examples of printed circuits. (a) Sinusoidal patterns printed at recommended parameters and widths of 1, 1.2, and 1.5 mm, respectively; (b) close up images of the circuits in (a) taken using the printer’s fiducial camera.

Figure 6

Steps for developing a fully stretchable sensor. (a) Sinusoidal silver conductive lines are printed on top of plasma-treated PDMS; (b) a protective coating of PDMS ink is inkjet-printed on top of the lines, and non-hazardous liquid metal (EGaIn, Sunnyvale, CA, USA) is incorporated for purposes of power transmission and signal acquisition; (c) flexible copper wires are inserted in liquid metal; (d) surface mount LEDs and PD are loaded on top of liquid metal; (e) uncured PDMS is poured to form a second layer of PDMS covering the circuit, optoelectronics, and wire connections.

Figure 7

Sinusoidal conductive patterns printed on PDMS at various amplitudes and line widths.

Figure 8

Average normalized resistances of two sinusoidal samples at a line width of 1 mm and various amplitudes as the applied strain increases (loading) and decreases (unloading).

Figure 9

Average normalized resistances of two sinusoidal samples at a line width of 1.2 mm and various amplitudes as the applied strain increases (loading) and decreases (unloading).

Figure 10

Average normalized resistances of two sinusoidal samples at a line width of 1.5 mm and various amplitudes as the applied strain increases (loading) and decreases (unloading).

Figure 11

Sinusoidal conductive patterns printed on PDMS at various numbers of cycles.

Figure 12

Average normalized resistances of two sinusoidal samples at various cycles as the applied strain increases (loading) and decreases (unloading).

Figure 13

Horseshoe conductive patterns printed on PDMS at various angles.

Figure 14

Average normalized resistances per unit length of two horseshoe samples at various angles as the applied strain increases (loading) and decreases (unloading).

Figure 15

Average resistances (Ω/mm) for a sinusoidal pattern and a horseshoe pattern at A = 4 mm, W = 1.5 mm, and four cycles as the applied strain increases (loading) and decreases (unloading).

Figure 16

Normalized resistance (R/R₀) for the optimal horseshoe pattern as it is cycled up to 3000 cycles at strains of 5, 10, and 20%.

Figure 17

Steps for developing a fully stretchable sensor. (a) 2 layers of sinusoidal silver conductive lines printed on top of plasma-treated PDMS; (b) 15 layers of transparent PDMS ink printed to coat the silver lines, which had a resistance of 4.7 Ω; (c) non-hazardous liquid metal (EGaIn, Sunnyvale, CA, USA) incorporated for purposes of power transmission and signal acquisition; (d) surface mount LEDs and PD loaded on top of liquid metal, and flexible copper wires inserted in liquid metal; (e) uncured PDMS added using a syringe into a mold to form a second layer of PDMS covering the circuit, optoelectronics, and wire connections; (f) final sensor prototype.

Figure 17

Steps for developing a fully stretchable sensor. (a) 2 layers of sinusoidal silver conductive lines printed on top of plasma-treated PDMS; (b) 15 layers of transparent PDMS ink printed to coat the silver lines, which had a resistance of 4.7 Ω; (c) non-hazardous liquid metal (EGaIn, Sunnyvale, CA, USA) incorporated for purposes of power transmission and signal acquisition; (d) surface mount LEDs and PD loaded on top of liquid metal, and flexible copper wires inserted in liquid metal; (e) uncured PDMS added using a syringe into a mold to form a second layer of PDMS covering the circuit, optoelectronics, and wire connections; (f) final sensor prototype.

Figure 18

Testing stretchable sensor functionality. (a) Red LED is turned ON; (b) infrared LED is turned ON; (c) PD signal in response to ambient light is acquired; (d) red LED is ON when sensor is bent around a concave curvature.

Figure 19

External circuitry. (a) Schematic diagram of external circuitry used to control the LEDs and acquire the PD signal; (b) PCB design of the external circuitry.

Figure 20

Functional stretchable pulse oximeter. (a) The stretchable pulse oximeter is interfaced with external circuitry; (b) data is collected by the sensor displayed on Arduino serial monitor and sensor output is compared to a commercial pulse oximeter.