The Internet of Things (IoT) involves several new concepts of network connectivity between physical objects and end users, motivating investigations into new tactile sensors and sensing mechanisms [
61]. Flexible pressure sensors and tactile sensors are considered the core of wearable electronics, particularly health-monitoring devices [
62]. Therefore, there is great demand for flexible and bendable materials. In this subsection, we discuss inkjet-printed tactile sensors fabricated on flexible substrates (polymer, plastic, paper and textile).
3.1.1. Inkjet-Printed Tactile Sensors on Polymer Substrates
Inkjet/aerosol-jet printed capacitive touch sensors were developed on two-dimensional and three-dimensional polymer substrates [
63]. Ag nano-ink (EMD5714, SunChemical, Carlstadt, NJ, USA) was used to print an interdigital capacitor on a thermoplastic substrate (polybutylene terephthalate). The researchers evaluated the touch sensor with a finger (2 s per touch) and detected the change in the capacitance of the two electrodes.
A passive LC resonator on a flexible substrate (polyethylene terephthalate (PET)) was proposed as a touch sensor [
38]. Ag nanoparticle ink (Metalon JS-015, NovaCentrix, Austin, TX, USA) was inkjet-printed to realize a planar LC resonator. An inter-digitated capacitive pattern (L × W = 1.5 × 0.8 cm) served as a sensing element with conductive tracks 200 nm thick, as shown in
Figure 2. The large size of the reader circuitry (because it was designed for low frequencies of 76 and 146 KHz, as the resonance frequencies correspond to 90 and 60 mm) was its drawback. Nevertheless, its contactless feature and passive readout circuitry make it an attractive choice for battery-less environments.
The sensing floor, which is an interesting application of tactile sensing, was introduced [
64]. A sheet of tiles is formed by inkjet-printing a Cu pattern on a flexible plastic substrate, as shown in
Figure 3. Four electrodes (120 × 120 mm) for capacitive sensing and two RF antennas (one for cellular GSM detection and another for near-field communication sensing) were printed. The footstep patterns were detected by electrodes embedded in the proposed sensing floor (passive capacitive sensing mode).
A flexible capacitive pressure sensor with an excellent but fixed sensitivity is proposed [
65]. Ag ink (Sigma–Aldrich, St. Louis, MO, USA) was inkjet-printed (DMP 2831, Dimatix Fujifilm, Santa Clara, CA, USA) on a flexible substrate (Arylite, 200 μm thick, Ferrania Corp., Liguria, Italy). A PMMA solution [mixture of PMMA (Sigma–Aldrich) dissolved in propylene glycol methyl ether acetate (PGMEA)] or a poly(vinylpyrrolidone) (PVP) solution [mixture of PVP (Sigma–Aldrich) and poly (melamine-co-formaldehyde) dissolved in PGMEA] was spin-coated on the Ag-printed Arylite substrate to form a dielectric layer. To fabricate pressure sensors, a multiscale-structured electrode was laminated on the top of the dielectric layer and a thin PET film (Toray Corp., Chuo-ku, Japan) was attached on top of the multiscale-structured PDMS to reduce the adhesiveness of the PDMS during the pressure-sensing tests. To perform measurements and characterization, a Cu wire was connected to both electrodes using Ag epoxy. Fingertip grip sensing was characterized, as shown in
Figure 4.
One year later, the same research group proposed another flexible pressure sensor with a tunable sensitivity, which introduced new avenues for versatile applications (similar to the behavior of human skin) [
62]. Ag nanowires (AgNW ink; SLV-NW-90, Blue Nano Inc., Cornelius, NC, USA) were spray-coated on a buckled mold PDMS. Three different formation ratios of the PDMS crosslinker were poured on the AgNW film to form a nanocomposite. The bottom plane was formed using an inkjet-printed Ag electrode on a poly(ethylene2, 6-naphtharate) (PEN) substrate. To perform a bending test, AgNW-embedded PDMS was affixed to the bottom side using Kapton tape and force was applied to the sensor using a bending-test machine. An IMADA force test bed was used to precisely apply vertical pressure using metal and/or plastic weights. A successful characterization of the microstructured AgNW-embedded PDMS confirmed the wavy structure of the nanowire composites, which was independent of the mixing ratio of the PDMS matrix. However, the shape dependence of the buckled structure on the PDMS mixing ratio was confirmed and the nanowire composite with a 10:1 mixing ratio exhibited the greatest change in its relative capacitance value. The fabrication steps and final layout of the capacitive flexible pressure sensor are shown in
Figure 5.
“PrintSense” is an interesting sensing device capable of multimodal flexible interaction [
66]. It is effective in various sensing scenarios, such as touch and hover-based as well as curved and deformable interfaces. It is a hybrid structure consisting of an inkjet-printed array of interdigitated electrodes (single layer) on a flexible substrate and customized circuitry to process electrode signals generated by finger touches, as shown in
Figure 6. To increase the detection range of proximity capacitive sensing, two additional sensing modes were included, as shown in
Figure 7.
A capacitive force sensor, i.e., a parallel-plate capacitor with PDMS (Sylgard 184, Dow Corning, Wiesbaden, Germany) as the dielectric, was proposed for artificial skin [
67]. One electrode was realized using Kapton (Pyralux, DuPont, Wilmington, DE, USA), with the top surface covered by a Cu laminated sheet. The other electrode was inkjet-printed using a solution of Ag nanoparticle ink (DGP 40LT-15C, ANP Co., Pleasanton, CA, USA) dissolved in triethylene glycol monoethyl ether. They characterized different thicknesses of the PDMS dielectric as well as the printed layers and analyzed the PDMS swelling phenomenon (coffee-ring effect). The change in the capacitance divided by the change in the force was defined as the sensitivity and its highest value was 3 pF/N.
Piezoresistive artificial skin capable of tactile sensing was proposed [
3]. PEDOT: PSS ink (CleviosTM, Heraeus Precious Metals GmbH, Hanau, Germany) was inkjet-printed (JETLAB 4-XL, Microfab Technologies, Inc., Plano, TX, USA) on a Cu-PDMS composite substrate. The conductivity of inkjet-printed patterns may suffer under elongation/bending. Secondly, the weak adhesion between the polymeric-based functional material and the metalized electrode may lead to peeling/detachment effects. Conventional plasma treatment cannot be employed to overcome these detrimental effects; therefore, the optimized direct patterning of electrodes on an already plasma-treated composite material is realized.
The fabrication of an instant inkjet-printed flexible sticker using commercially available inks and a conventional office printer was proposed [
59]. An interdigitated capacitive electrode functions as a sensor and can detect even the exact touch location when a slight change in the effective permittivity near its interface is induced. The accessible, low-cost and rapid printing technique is claimed to be viable for designing custom shape patterns on both flexible substrates (polymer and paper) and conventional (rigid) substrates.
Low-cost polymer substrates having a temperature limitation (approximately 200 °C) are unsuitable for high-performance flexible electronics. To overcome this issue, technical collaboration was initiated between ORNL and NovaCentrix [
68]. An Ag/Cu Metalon ink (NovaCentrix)-based inkjet-printed capacitive touchpad is realized on various temperature-sensitive flexible substrates (PET and polyimide). The novelty was the unique photonic curing process (using PulseForge
® tools) to heat the thin films with the high-intensity, short-duration (250 μs) light pulses required for thin-film densification, recrystallization and annealing. This process is selective to inkjet-printed patterns and does not damage the main substrate material or other integrated circuits.
3.1.2. Inkjet-Printed Tactile Sensors on Paper Substrates
Photo paper offers remarkable advantages as a sensor-substrate, which were presented in
Section 2. Here, we discuss several paper-based tactile sensors fabricated using inkjet-printing technology.
In [
69], an inkjet-printed touchpad based on spiral resonators is proposed. Its principle phenomenon relies on the detection of changes in the resonance frequency of the spiral resonator due to a finger approaching the touchpad and causing electromagnetic coupling. It can detect one or two fingers even if the fingers are in close proximity of the sensor surface. The touchpad was designed on Kodak photo paper, using ANP Ag Jet 55 LT-25C Ag nanoparticle ink with a Dimatix DMP-2831 printer, as shown in
Figure 8. To ensure a high conductivity, three layers of Ag nanoparticles were printed and then sintering (2 h at 120 °C) was conducted. The measured S21 shows that distinctive resonance frequencies were obtained when both spirals touched individually and simultaneously, as shown in
Figure 8d.
In [
70], a multi-key touch interface based on resistive touch sensing is proposed on photographic paper. Voltage-divider circuitry is in action when a finger is brought in contact and it acts as a close switch, which otherwise remains open. The researchers evaluated the touchpad with 10, 15 and 20 touch contacts, yielding an accuracy of 99%, 93% and 91%, respectively. A touch detection rate of 154 touches per minute was reported. They used an Epson WF30 printer, along with NBSIJ-MU01 ink and paper from Mitsubishi Imaging (Rye, NY, USA).
In [
71] a multi-touch capacitive sensor is proposed, which can be customized in shape and size. They used Ag conductive ink and an inkjet printer to print conductive layers and electrodes on paper. They demonstrated their touchpad idea using various topologies and sizes and demonstrated the robustness of the sensing device even after it was cut.
In [
72], an interactive paper serving as a paper-based touch sensor via inkjet printing and conductive ink is proposed. The complete design process—from design and printing to system integration—is described and one can fabricate it without knowing technical details. The researchers also presented a recording option and logic demonstration; however, the design process was lengthy and slow.
A capacitive touchpad consisting of nine touch buttons is proposed, which can detect a change in the effective capacitance when the impedance is altered by a finger or touch pen in contact with the sensor surface [
73]. An EPSON printer was used to print Ag nanoparticle ink (Metalon JS-B25P, Novacentrix, Austin, TX, USA) on a photo paper (Kodak, Rochester, NY, USA). They evaluated the sensor performance using a test setup (ARDUINO board, a demultiplex chip and an amplifier) and reported the distinguished capacitance values as 288–236 and 340–564 pF for the untouched and touched states, respectively. The touchpad exhibited a height sensitivity of 1.88 mm
−1 for h < 0.2 mm and an area sensitivity of 0.02082 mm
−2 for A < 35 mm
2.
A wireless touch and proximity sensor is proposed [
74]. It consists of two inkjet-printed antennas (one for sensing and one for reference), as shown in
Figure 9. The matching of the sensing antenna changes when a finger is touched or comes in proximity. The sensing and calibration antenna exhibited a similar return loss and a relative shift of 60 MHz was observed when a touch event is detected.
A capacitive touch control pad was fabricated via a hybrid approach on a paper, kapton and glass substrate [
75]. Here, a Dimatix DMP-2831 printer was used to inkjet-print 40 wt. % Ag nanoparticle ink (Silverjet DGP HR, ANP, Bugang-myeon, Sejong, Korea) with a 10-pL cartridge on paper (Powercoat 230, Arjowiggins, Stamford, CT, USA) and polyimide Kapton (ISOAD TAPE 7004, DuPont, Wilmington, DE, USA). Nozzle platen (at 40 °C) was selected to assist the vaporization of the volatile solvent, yielding the best droplets on the surface. Three different materials were tested to connect surface mount devices to the inkjet-printed pads: a Silverjet ink compound, which is also used for the inkjet-printed pattern; Ag epoxy (CircuitWorks Conductive Epoxy CW2400, Chemtronix, Kennesaw, GA, USA); and solder flux (Qualitek, Sn 97% Cu 3%). All three soldering materials were characterized on each substrate and evaluated with regard to the lowest contact resistances: solder for Kapton with 0.12 Ω and AgNP for paper and glass with 0.27 Ω and 0.37 Ω, respectively.
It is not simple to integrate buttons with a touch sensor/screen. Until 2012, there was only one patent for capacitive buttons on posters, which describes a methodology for receiving input/keystrokes on paper-based substrates. However, some research articles (for instance, [
69]) discuss the effect of adding one or more extra layer(s) on top of paper-based touch sensors and characterize the touch response. In [
76], the researchers characterized two different types of capacitive buttons in relation to their usage on a paper-based keypad (with 10 individual keys).