# Programmable Organic Chipless RFID Tags Inkjet Printed on Paper Substrates
## 3 Materials and Methods  
Several works report the development of chipless RFID tags by using metallic inks because of their inherently good electrical conductivity [60; 61; 62]. More recently, conducting polymers such as poly (3,4-ethylenedioxythiophene) polystyrene sulfonate, hereafter referred to as PEDOT:PSS, have been widely employed in capacitors, transistors and solar cells [63; 64; 65]. Indeed, PEDOT:PSS possesses interesting features due to its ease of processing, mixed electronics/ionic conductivity and biocompatibility. Using PEDOT:PSS is a challenge since organic conductors usually exhibit conductivities from 2 to 3 times lower than those of metallic inks. In order to fabricate organic chipless RFID tags, PEDOT:PSS doped with multi-walled carbon nanotubes (named Poly-Ink HC) was purchased from Poly-Ink. In this study, the rheological property of the conductive ink is suitable for drop-on-demand (DoD) inkjet printing given that its viscosity is between 3-8 mPas. The ink was ultrasonically stirred for 1 min and then filtered using a 0.2 \(\upmu\)m PVDF filter after which the tags were printed onto a standard DIN A4 photocopy paper using an inkjet printer (Ceradrop Cera-Printer X-Series). The printhead (cartridge) had 16 nozzles each capable of ejecting an ink droplet of 10 pL. As the tag dimensions require a considerable resolution, a single nozzle was selected for droplet ejection for preventing an excess of ink which could produce a short-circuit in S1 and S2 owing to the capillarity of the cellulose. During the inkjet printing process, both the substrate and the cartridge were kept at room temperature. The geometry of the printed resonators of the tags was identical to that of the resonator of the sensitive part of the reader (see Figure 1). Note that despite the fact that the resonators of the tag were printed on DIN A4 (a different substrate than the resonator of the reader), system functionality is guaranteed. The reason is that the shift in the frequency response (when a resonant element of the tag is on top of the resonator of the reader) is mainly determined by the geometry of such elements, and this is optimum when the geometries of both elements are  
Figure 1: Layout of the sensitive part of the reader. Dimensions (in mm) are: \(l_{1}=3.16\), \(l_{2}=3.35\), \(S_{1}=0.2\), \(S_{2}=0.2\), \(W_{1}=0.56\), and \(W_{2}=0.5\). Figure 2: Simulated response of the reader without and with resonator on top of it. The considered substrate of the reader is Rogers RO3010 with dielectric constant \(\epsilon_{\tau}=10.2\), thickness h = 1.27 mm, and loss tangent tan\(\delta\) = 0.0022. For the tag, the considered substrate is Rogers RO4003C with dielectric constant \(\epsilon_{\tau}=3.55\), thickness h = 0.20 mm, and loss tangent tan\(\delta\) = 0.0021. SRR stands for split ring resonator. The air gap is considered to be 0.25 mm. Reprinted with permission from [59]. identical. Nevertheless, it does not mean that the frequency shift does not depend also on the substrate. It does, but a small shift, as in Figure 2, suffices to differentiate the presence or absence of functional resonant elements in the tag, the principle for tag reading. Prior to tag fabrication, it was necessary to ensure a proper overlap of the droplets and conductivity. Depending on the selected space between adjacent droplets, a printer can eject isolated drops or produce scalloped, uniform, or bulging lines [66]. To form a continuous and conducting film, the drop spacing needs to be properly spaced with partial overlap. By carefully optimizing the drop spacing, it is possible to achieve higher resolutions with even edges. Figure 3a illustrates the printed line topologies across a broad range of drop spacing. As drop spacing is increased above a certain length, the resultant line changes from a scalloped one (for drop spacing 5-10 \(\upmu\)m) to a continuous line (drop spacing 20 \(\upmu\)m) and then gradually separates into isolated drops (drop spacing over 50 \(\upmu\)m).