Inkjet printing is a relatively new technology for low temperature thin film depositions on planar substrates. This paper presents a number of wireless biosensor prototypes developed using this technology. The thin films were developed with a material deposition printer (Dimatix DMP-2831, FujiFilm Dimatix Inc., NH) on various flexible substrates including paper. The printed thin films were silver nanoparticles (Metalon JS-B25HV, Nanosilver Ink, Novacentrix, TX), copper oxide nanoparticles (Metalon ICI-002, Copper oxide Ink, Novacentrix, TX), and polypyrrole (Sigma Aldrich Co. LLC, St. Louis, MO). The average sizes of these nanoparticles were between 85 to 115 nm. The printing ink viscosities were between 3 to 10 cPa with 30% to 50% loading. The thicknesses of the silver nanoparticle films were in the range of 250 to 350 nm. The drop spacing was kept at 15 µm by setting the printer resolution to 1693 dpi. The trace widths for electrically conductive silver nanoparticle thin film were between 100 to 500 µm.

To manipulate track resistances or increase the thickness of insulation layers, multiple coatings of overlaying thin film deposition can be effectively employed, provided that a new coating is applied after the previous coating is fully cured. Multilayer circuits were also developed using copper oxide insulation layer sandwiched between two silver thin film traces. Vias were created by not printing any insulation layer in the via region, and applying multiple coatings of silver traces only inside these via cavities. The top surface irregularity of the insulation layer was minimized by limiting the droplet sizes using drop rates between 20 to 25 kHz.

This thin film printing technology was utilized to prototype fully passive wireless sensors for physiological biosignal monitoring with our proprietary technology - Resistive Wireless Analog Passive Sensors (rWAPS). The rWAPS circuits enjoy battery-less operation, low component count, fast response time, and low RF power requirement and is suitable for this printing technology. The prototypes were developed on flexible substrates as low-cost body-worn disposable electronic patches. The interrogator probes the sensors wirelessly at 13.65MHz using inductive coupling principle, and the resistive transducer at the sensors modulates the amplitude of the reflected wave, which can be decoded with envelope detectors. Ability to develop thin film multilayer printed circuits on flexible substrates coupled with the simplicity of the rWAPS system demonstrate the possibility of wear-and-forget type body-worn wireless biosensors to routinely monitor physiological signals for a multitude of biomedical applications.