Mohammadmahdi Faraji

Inkjet is a versatile non‐contact printing technique that uses droplets of ink jetted from nozzles to print on a variety of substrates. Although some modern piezoelectric printheads support high viscosity fluids (>10 cP), they are currently expensive and may not be suited for many benchtop experiments in research environment. Here, the design, fabrication by 3D‐printing (no cleanroom processes or silicon micromachining are involved) and testing of a multi‐purpose single‐nozzle piezoelectric dispenser/printhead for medium‐viscosity fluids (10–60 cP) is reported. Custom control electronics are developed to drive the printhead. The design features a 200 μm diameter nozzle, suitable for nL drop generation with solutions containing nm sized particles, thus allowing the deposition of μm thick layers of functional inks in a single printing pass. The inkjet dispenser is first tested with glycerol solutions, resulting in drops of 1.9–3.6 nL with typical drop velocity of 0.5–1.3 m s⁻¹. The developed piezoelectric dispensing system is then demonstrated in the printing of a two‐layer capacitive sensor, using silver nanoparticle ink (conductive ink) and a transparent encapsulant ink (dielectric ink). The reported piezoelectric dispenser design can be optimized for many research purposes, allowing the dispensing of functional chemicals, materials, and biomolecules.

The growing demand for continuous cardiac signal monitoring has propelled the development of highly sensitive flexible sensors with reduced power requirements. Proposed flexible ferroelectrets/piezoelectrets are voided polymers that can be polarized/poled, acquiring a piezoelectric behavior that is suited for sensing applications. Here, ferroelectret preparation was achieved by laser-patterning of an adhesive-coated polypropylene (PP) spacer with low Young’s modulus, resulting in improved piezoelectric coefficient while avoiding high-temperature thermal bonding between the different ferroelectret layers. The spacer pattern and thickness were optimized and compared to a suitable analytical model. The optimized piezoelectret showed a low compressive modulus (~300 kPa), a maximum dynamic piezoelectric coefficient d ₃₃ = 81 ± 5 pC/N and a quasi-static piezoelectric coefficient of ~8552 pC/N. In the short term, the sensors were stable in a wide range of temperature and relative humidity, but after 70 days at room temperature the d ₃₃ decreased to ~36 % of the initial value. Finally, the optimized ferroelectret device was demonstrated in the measurement of the human pulse and extraction of cardiovascular parameters (heart rate and carotid-femoral pulse wave velocity).

As an additive manufacturing process, material jetting techniques allow to selectively deposit droplets of materials in liquid or powder form through a small-diameter aperture, such as a nozzle of a print head. For the fabrication of printed electronics, a variety of inks and dispersions of functional materials can be deposited by drop-on-demand printing on rigid and flexible substrates. In this work, zero-dimensional multi-layer shell-structured fullerene material, also known as carbon nano-onion (CNO) or onion-like carbon, is printed on polyethylene terephthalate substrates using drop-on-demand inkjet printing. CNOs are produced using a low-cost flame synthesis technique and characterized by electron microscopy, Raman, x-ray photoelectron spectroscopy, and specific surface area and pore size measurements. The produced CNO material has an average diameter of ∼33 nm, pore diameter in the range ∼2–40 nm and a specific surface area of 160 m².g⁻¹. The CNO dispersions in ethanol have a reduced viscosity (∼1.2 mPa.s) and are compatible with commercial piezoelectric inkjet heads. The jetting parameters are optimized to avoid satellite drops and to obtain a reduced drop volume (52 pL), resulting in optimal resolution (220 μm) and line continuity. A multi-step process is implemented without inter-layer curing and a fine control over the CNO layer thickness is achieved (∼180 nm thick layer after 10 printing passes). The printed CNO structures show an electrical resistivity of ∼600 Ω.m, a high negative temperature coefficient of resistance (−4.35 × 10⁻² °C⁻¹) and a marked dependency on relative humidity (−1.29 × 10⁻² RH_% ⁻¹). The high sensitivity to temperature and humidity, combined to the large specific area of the CNOs, make this material and the corresponding ink a viable prospect for inkjet-printed technologies, such as environmental and gas sensors.

Texture detection is one of the essential features requested for artificial tactile sensing to push the demand for flexible low-cost tactile sensors in the robotics sector. In this manuscript, we demonstrate the ability of a ferroelectret-based pressure sensor together with a patterned elastomer layer to detect surface textures. The ferroelectret sensor was fabricated using fluorinated ethylene propylene (FEP) sheets bonded with a patterned adhesive layer to create cavities, integrated with the elastomer bumped surface, and finally charged using a pyroelectric method developed by our group. The ferroelectret-based sensor showed a linear response to the applied force in the range of 0.5 to 2 N, a piezoelectric coefficient of 150.1 ± 3.2 pC/N in the range of 10–80 Hz, and a flat dynamic response in the range of 10–1000 Hz. The tactile sensing characterization of the sensor, performed at different scanning speeds (10 to 30 mm/s) and gratings with different periodicities (0 to 0.8 mm), showed that the fundamental frequencies observed ranged from 12 Hz to 75 Hz, as expected from the model. These results lay the foundation for the adoption of such sensors in different applications that need fine tactile information, such as an autonomous or teleoperated robotic hand, prostheses, and wearable devices.

In this article, a pyroelectrically charged fluorinated ethylene propylene (FEP) piezoelectret force sensor is reported. The proposed pyroelectric charging technique provides reproducible and tunable charge generation. Pyroelectric charging is simple and compact when compared to other ferroelectret charging methods, such as corona discharge and dielectric barrier discharge and irradiation, which require high-voltage sources, sharp tips, and vacuum setups. In this pyroelectric charging study, temperature ramp rates of 1 °C/s ( dT $=15^{\circ }\text{C}$ ), 2 °C/s ( dT $=30^{\circ }\text{C}$ ), and 4 °C/s ( dT $= 60\,\,^{\circ }\text{C}$ ) are applied to a lithium niobate pyroelectric crystal using a programmable temperature control system. The effects of the temperature ramp applied to the pyroelectric crystal and the number of temperature ramps on the piezoelectret charging are assessed. The pyroelectrically charged FEP piezoelectrets show ${d}_{{33}}$ piezoelectric coefficients up to 68.6 pC/N. The output of the FEP sensors is linear with the force and independent of frequency in the range of 45–505 Hz. The charge retention was assessed, showing no degradation of the piezoelectric activity up to 66 h. Finally, the FEP ferroelectret sensors were used in a pressure sensor array, allowing the recording of a human step (gait analysis experiment).

In this manuscript, a pyroelectrically-charged flexible polydimethylsiloxane (PDMS) piezoelectret force sensor is reported. The proposed pyroelectric charge trapping approach supports low power operation, small footprint, uniform charge distribution, and chip-scale integration for recharging capability. Various flexible porous PDMS membranes (150 mm in diameter, ~100 µm-thick) are fabricated using a water-in-PDMS emulsion technique for controlled pore size (0.6 – 142 µm) and their piezoelectric coefficients are measured to be 70–170 pC/N, representing increases of 100% to 450% after charging. Finally, the charged membranes were used as a pressure sensor array, allowing the recording of a human step.