Review of Recent Inkjet-Printed Capacitive Tactile Sensors

Abstract

Inkjet printing is an advanced printing technology that has been used to develop conducting layers, interconnects and other features on a variety of substrates. It is an additive manufacturing process that offers cost-effective, lightweight designs and simplifies the fabrication process with little effort. There is hardly sufficient research on tactile sensors and inkjet printing. Advancements in materials science and inkjet printing greatly facilitate the realization of sophisticated tactile sensors. Starting from the concept of capacitive sensing, a brief comparison of printing techniques, the essential requirements of inkjet-printing and the attractive features of state-of-the art inkjet-printed tactile sensors developed on diverse substrates (paper, polymer, glass and textile) are presented in this comprehensive review. Recent trends in inkjet-printed wearable/flexible and foldable tactile sensors are evaluated, paving the way for future research.

Keywords: capacitive sensing; flexible polymer; glass; inkjet printing; paper; tactile sensors.

Figure 1

Principle of capacitive sensing.

Figure 2

Inkjet-printed LC resonator on a PET substrate, serving as a touch sensor (redrawn from [38]): (a) layout; (b) complete design of the touch-sensor system (printed LC sensing block coupled to the printed circuit board (PCB)-based contactless, passive reader).

Figure 3

(a) A single sensing tile consists of the substrate along with a PCB module (redrawn from [64]). Four different electrodes in each sensing tile transmit four unique signals during footstep detection; (b) Demonstration of the folding mechanism for adapting the surface geometry using a single substrate without any cutting or joining.

Figure 4

Fingertip grip pressure-sensing device (redrawn from [65]). (a) Fingertip grip pressure sensors attached to four fingertips; (b,c) Holding a plastic beaker with four fingertips; the resultant relative capacitance changes are indicated.

Figure 5

Fabrication process for the capacitive flexible pressure sensor with a nanowire composite and a bottom plane (redrawn from [62]). (a) Fabrication process for nanowire composites with different mixing ratios of the PDMS matrix; (b) Fabrication process for the bottom plane: the inkjet printing of the Ag electrode onto the PEN substrate and the spin coating of the PVP dielectric layer; (c) Layout of the final product.

Figure 5

Fabrication process for the capacitive flexible pressure sensor with a nanowire composite and a bottom plane (redrawn from [62]). (a) Fabrication process for nanowire composites with different mixing ratios of the PDMS matrix; (b) Fabrication process for the bottom plane: the inkjet printing of the Ag electrode onto the PEN substrate and the spin coating of the PVP dielectric layer; (c) Layout of the final product.

Figure 6

PrintSense: a new development in multi-modal sensing arrays, printed as a single layer on a flexible substrate and connected to a customized hardware module (left). Capacitive touch sensing, proximity sensing, galvanic skin response (i.e., resistive sensing) and curved sensing are supported (redrawn from [66]).

Figure 7

PrintSense: Three different signal-detection schemes (redrawn from [66]). From top to bottom: active transmit-and-receive capacitive sensing for proximity and folding detection, alternating-current hum detection for touch and proximity detection and galvanic skin response.

Figure 8

Touchpad: inkjet-printed spiral resonator on photo paper (redrawn from [69]). (a) Fabricated prototype of the touchpad (b) when one spiral is touched, (c) side view of the bent touchpad at a specific curvature ratio and (d) measured S21 when both spiral resonators are touched.

Figure 9

RFID touch sensor (redrawn from [74]): (a) layout of chip-based touch sensor and (b) fabricated prototype of inkjet-printed RFID touch sensor.

Figure 10

Top view of the fabric-based all-inkjet-printed flexible capacitor: (a) schematic view generated using L-Edit software; (b) fabricated prototype (redrawn from [25]).

Figure 11

(a) Schematic of a fully inkjet-printed transparent touch sensor, a grid of PEDOT:PSS (x-axis and y-axis) and a poly(methylsiloxane) dielectric separator fabricated via inkjet printing; (b) Characterization of a touch sensor with the absence of touch and a single touch at the center (redrawn from [42]).