applied sciences Article Programmable Organic Chipless RFID Tags Inkjet Printed on Paper Substrates Miquel Moras 1 , Carme Martínez-Domingo 1 , Roger Escudé 1 , Cristian Herrojo 2 , Ferran Paredes 2 , Lluís Terés 1 , Ferran Martín 2 and Eloi Ramon 1, * 1 Institut de Microelectrònica de Barcelona, IMB-CNM (CSIC), 08193 Bellaterra, Spain; miquel.moras@imb-cnm.csic.es (M.M.); cmartinez3@icmab.es (C.M.-D.); roger.escude@imb-cnm.csic.es (R.E.); lluis.teres@imb-cnm.csic.es (L.T.) 2 CIMITEC, Departament d’Enginyeria Electrònica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain; cristian.herrojo@uab.cat (C.H.); ferran.paredes@uab.cat (F.P.); ferran.martin@uab.cat (F.M.) * Correspondence: eloi.ramon@imb-cnm.csic.es Abstract: In this paper, an organic, fully recyclable and eco-friendly 20-bit inkjet-printed chipless RFID tag is presented. The tag operates in the near field and is implemented by means of chains of resonant elements. The characterization and manufacturing process of the tag, printed with a few layers of a commercial organic ink on conventional paper substrate (DIN A4), are presented, and tag functionality is demonstrated by reading it by means of a custom-designed reader. The tags are read by proximity (through the near field), by displacing them over a resonator-loaded transmission line,   and each resonant element (bit) of the tag is interrogated by a harmonic signal tuned to the resonance frequency. The coupling between the reader line and the resonant elements of the tag produce and Citation: Moras, M.; amplitude modulated (AM) signal containing the identification (ID) code of the tag. Martínez-Domingo, C.; Escudé, R.; Herrojo, C.; Paredes, F.; Terés, L.; Keywords: chipless RFID; organic inks; inkjet printing; printed electronics; secure paper; Martín, F.; Ramon, E. Programmable Organic Chipless RFID Tags Inkjet authentication Printed on Paper Substrates. Appl. Sci. 2021, 11, 7832. https://doi.org/ 10.3390/app11177832 1. Introduction Academic Editor: Oscar Paper electronics is a broad area of flexible electronics with the focus on paper and Reinoso García cellulose-based materials not only as a substrate, but also as functional layers for electronic devices [1–6]. Despite the high porosity and roughness of paper, its use reduces production Received: 28 July 2021 costs and finds applications in many electronic devices, such as thin film transistors (TFT), Accepted: 23 August 2021 passive electronic components, energy-storage devices, and MEMS [1,5,7,8]. TFTs are one Published: 25 August 2021 of the most basic, and yet, most important elements of modern-day electronics [9–12]. Inkjet printing is a known deposition technology for manufacturing devices in the field of Publisher’s Note: MDPI stays neutral flexible and printed electronics [13–17]. The technology offers various advantages, such with regard to jurisdictional claims in as additive printing process ability, accuracy in the micrometer range, and flexibility in published maps and institutional affil- terms of material processing [17–21]. Due to its advantages, the technology has begun to iations. replace traditional metal subtractive etching/lithographic as well as sputtering/e-beam evaporation technologies in large-area and flexible substrates [22,23]. These technologies are expensive, complicated, need specific equipment, and impose strict process require- ments. In contrast, depending on the field of application, paper-based electronics can be Copyright: © 2021 by the authors. manufactured using photolithography, screen printing, gravure printing, flexography, or Licensee MDPI, Basel, Switzerland. direct-writing/-printing technologies [21,24–29]. In these mentioned printing techniques, This article is an open access article inks generally have higher viscosity, and this property restricts them from unwanted ink distributed under the terms and diffusion into the paper fibbers [24,30]. conditions of the Creative Commons In this study, we promote the use of inkjet printing for fabricating chipless RFID Attribution (CC BY) license (https:// tags on paper substrates as a development of inkjet technology already used in industrial creativecommons.org/licenses/by/ manufacturing. In the last decade, several research groups have actively contributed to 4.0/). Appl. Sci. 2021, 11, 7832. https://doi.org/10.3390/app11177832 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 7832 2 of 12 the development of the active and passive devices on flexible polymeric substrates, e.g., polyethylene terephthalate, polyethylene naphthalate (PEN), and polyimide films, among others. Some of the examples are inkjet-printed antennas, TFT arrays, capacitors and logical circuits among others [31–38]. However, as regards printed chipless RFID tags manufactured on paper substrate, only a few examples have been reported [39,40]. Chipless Radio Frequency Identification (RFID) is a wireless technology used for identification (ID), tracking, sensing, and authentication/security applications [41,42]. In the field of authentication and security, one promising scenario for chipless RFID is secure paper. Within this particular application, equipping documents (e.g., banknotes, certificates, exams, ballots, official documents, etc.) with a planar ID code is envisaged as a means to fight against counterfeiting. The main general advantage of chipless RFID over chipped RFID systems is the absence of silicon integrated circuits, or chips, which in the case of chipless RFID tags are replaced with printed encoders. Such encoders can be fabricated by means of printing techniques, such as screen printing, rotogravure, flexography, or inkjet, and represent a low-cost solution as compared with conventional chip-based RFID tags [43–54]. However, chipless RFID tags present three main limitations: (1) data capacity, (2) tag size, and (3) shorter read ranges. These negative aspects and the fact that the materials (inks) and manufacturing processes (such as substrate functionalization and printing) necessary for tag fabrication are still not significantly cheaper than passive chipped RFID tags have limited the market penetration of chipless RFID technology. However, if only tag manufacture is considered, tag cost can be dramatically reduced by replacing ICs with encoders and the cost of mass-produced chipless RFID tags is predicated to fall below USD0.01 [43]. In our chipless RFID system described in a previous study [55] the tags are formed by chains of identical resonant elements printed or etched in one side of the substrate. These tags are read by proximity (through the near field), by displacing them over the sensitive part of the reader. The tag encoding is achieved by the presence or absence of certain resonant elements at predefined positions and is carried out by means of an interrogation harmonic signal tuned to the resonance frequency of the resonant elements. The presence or absence (resonator detuned or inoperative) of a resonant element will produce a variation of the amplitude of the interrogation signal at the output of the reader line. As a result, an amplitude modulated (AM) signal that contains the tag ID code is obtained. The tag ID code is inferred bit to bit, sequentially, by displacing the tag over the reader—a method of reading based on time division multiplexing [56]. 2. The Tags and the Reader As previously mentioned, near-field chipless RFID systems comprise the reader and the tag. The sensitive part of the reader consists of a microstrip transmission line loaded with a square-shaped split ring resonator (SRR) in bandpass configuration [57,58], as can be seen in Figure 1. When the tag, consisting of a set of identical resonators to that of the reader but oppositely oriented, is located in close proximity to the sensitive part of the reader, coupling between the resonators arises, and the overall response is shifted down. A large excursion of the transmission coefficient at the design frequency generates a high dynamic range at that frequency, i.e., a high difference between two transmission coefficient values. In Figure 2, the unloaded reader allows the signal to be transmitted (equal to the ‘1’ logic state); meanwhile, the reader loaded with the tag resonator blocks and reflects back the signal (equals to the ‘0’ logic state). Note that the simulated response depicted in Figure 2 is a general solution of a tag formed with one resonator element and considering a Rogers substrate with characteristics shown in this figure caption. Appl. Appl.Sci. 2021, Sci. 11,11, 2021, x FOR 7832 PEER REVIEW 3 of3 of 1212 Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 12 . . Figure Figure 1. 1. Layout Layout of the of the sensitive sensitive partpart of the of the reader. reader. Dimensions Dimensions (in mm) (in mm) are: lare: l1 = l3.16, 1 = 3.16, l2 =S3.35, 2 = 3.35, 1 = Figure 0.2, 1. SLayout S1S=2 =0.2, 0.2, of Wthe sensitive part=of the reader. Dimensions (in mm) are: l1 = 3.16, l2 = 3.35, S1 = 2W=1 0.2, = 0.56, 1and W2 and = 0.56, = 0.5.W 2 0.5. 0.2, S2 = 0.2, W1 = 0.56, and W2 = 0.5. . . Figure 2. Simulated response of the reader without and with resonator on top of it. The considered Figure 2. Simulated response of the reader without and with resonator on top of it. The considered substrate Figure of the reader 2.ofSimulated is Rogers RO3010 with dielectric andconstant εr = 10.2,on thickness top of it.hThe = 1.27 mm, and substrate the readerresponse of RO3010 is Rogers the reader withwithout dielectric with resonator constant εr = 10.2, thickness h = considered 1.27 mm, andloss tangent substrate loss tangent tanδ of the tanδ= 0.0022. reader For Forthe is Rogers = 0.0022. tag, tag,the RO3010 the considered with the substrate dielectric considered constantisis substrate εRogers = 10.2,RO4003C r Rogers thicknesswith RO4003C dielectric h = 1.27 with mm, dielec‐ constant and loss ε = tangent r 3.55, thickness tanδ = h 0.0022. = For0.20 the mm, tag, and the loss tangent considered tanδ = substrate 0.0021. is SRR Rogers stands RO4003C tric constant εr = 3.55, thickness h = 0.20 mm, and loss tangent tanδ = 0.0021. SRR stands for split ring for split with ring dielec‐ tric constant resonator. ε The = 3.55, air gapthickness is h = considered 0.20 to mm, be and 0.25 mm.loss tangent Reprinted tanδ with = resonator. The air gap is considered to be 0.25 mm. Reprinted with permission from [59]. r 0.0021. SRR permission stands from for [59]. split ring resonator. The air gap is considered to be 0.25 mm. Reprinted with permission from [59]. 3. Materials and Methods 3. Materials and Methods Severaland 3. Materials works report the development of chipless RFID tags by using metallic inks Methods Several works report the development of chipless RFID tags by using metallic inks because of Several their works inherently reportgood good the electrical conductivity development of chipless [60–62]. RFID MoretagsMore by recently, conducting using metallic inks because of their inherently electrical conductivity [60–62]. recently, conducting polymers because of such their as poly inherently (3,4-ethylenedioxythiophene) good electrical conductivity polystyrene [60–62]. More sulfonate, recently, hereafter conducting re- polymers ferred to such as as poly (3,4‐ethylenedioxythiophene) PEDOT:PSS, have been widely employed polystyrene in capacitors, sulfonate, hereafter transistors and re‐ solar polymers ferred to assuch as poly (3,4‐ethylenedioxythiophene) PEDOT:PSS, have been possesses widely employed polystyrene in capacitors, sulfonate, hereafter re‐ cells [63–65]. Indeed, PEDOT:PSS interesting features duetransistors to its easeand solar of process- ferred cells to as PEDOT:PSS, have been widely employed infeatures capacitors, transistorseaseand solar ing,[63–65]. Indeed, PEDOT:PSS mixed electronics/ionic possesses conductivity interesting and biocompatibility. dueUsingto itsPEDOT:PSS of pro‐ is a cells cessing, [63–65]. mixed Indeed, PEDOT:PSS electronics/ionic possesses conductivity interesting and biocompatibility. features due to Using2 to its ease PEDOT:PSS of pro‐ is challenge since organic conductors usually exhibit conductivities from 3 times lower a cessing, challenge mixed since electronics/ionic organic than those of metallic inks. conductors conductivity usually and biocompatibility. exhibit conductivities fromUsing 2 to 3PEDOT:PSS times lower is a challenge than those since of metallic organic conductors usually exhibit conductivities inks.organic chipless RFID tags, PEDOT:PSS doped with multi-walled from 2 to 3 times lower In order to fabricate thanInthose of to metallic inks. carbon nanotubes (namedorganic order fabricate Poly-Ink chipless HC) was RFID tags, PEDOT:PSS purchased from Poly-Ink. doped withstudy, In this multi‐ the walled Incarbon order nanotubes to fabricate(named organicPoly‐Ink chiplessHC) RFID was tags, PEDOT:PSS purchased from doped Poly‐Ink. withInmulti‐ this rheological property of the conductive ink is suitable for drop-on-demand (DoD) inkjet walledthe study, carbon nanotubes (named rheological Poly‐Ink HC) was purchased from Poly‐Ink. In this printing given that its property viscosity is ofbetween the conductive 3–8 mPas. inkThe is ink suitable for drop‐on‐demand was ultrasonically stirred for study, (DoD) the rheological property of the conductive ink is suitable for drop‐on‐demand 1 mininkjet and then printing given filtered usingthat its viscosity a 0.2 µm PVDFisfilter betweenafter 3–8which mPas.the Thetags inkwere was ultrasoni‐ printed onto a (DoD) cally inkjet stirred forprinting 1 minute given and that then its viscosity filtered using is between a 0.2 μm 3–8 PVDF mPas.filter The afterinkwhich was ultrasoni‐ the tags standard DIN A4 photocopy paper using an inkjet printer (Ceradrop CeraPrinter X-Series). callyprinted were stirred onto for 1 aminute standard andDINthenA4 filtered using paper photocopy a 0.2 μm using PVDFan filter after inkjet which printer the tags (Ceradrop The printhead (cartridge) had 16 nozzles each capable of ejecting an ink droplet of 10 pL. were As the printed CeraPrinter onto a standard tag X‐Series). dimensions The DIN printhead require A4 photocopy (cartridge) a considerable hadpaper using 16 nozzles resolution, aneach a single inkjet printer capable nozzle wasof (Ceradrop ejecting selected for CeraPrinter androplet ink droplet X‐Series). of 10 ejection forpL. The printhead As the tag preventing (cartridge) andimensions excess of ink had require 16 whichacould nozzles considerable each capable produceresolution, of a short-circuita ejecting single in S1 an andink nozzle S2 droplet was owing ofthe selected to 10for pL. As theejection droplet capillarity tagthe of dimensions requirean for preventing cellulose. During a considerable the excess inkjet of ink resolution, printing which could process, a single pro‐ both the nozzle duce was selected a short‐circuit substrate for droplet in S1 and S2 and the cartridge ejection owing were kepttoatthefor roompreventing capillarity temperature. an excess of the The of cellulose. ink geometry which During could pro‐ the printed of the inkjet duce printing a short‐circuit resonatorsprocess, of theboth inthe tags S1 andidentical S2 owing substrate was andtothetothat the capillarity cartridge of wereofkept the resonator the ofcellulose. at room Duringpart temperature. the sensitive the of inkjet The the printing geometry process, of the both printed the substrate resonators of and the the tagscartridge was were identical kept to reader (see Figure 1). Note that despite the fact that the resonators of the tag were printed that at room of the temperature. resonator of The the geometry sensitive on DINpart A4of (a thedifferent of printed the reader resonators (see Figure substrate of 1). than the tags Note the was despite that resonator identical of thethe to fact thatthat reader), ofsystem the theresonator resonators of of functionality the sensitive theis tag part were printed guaranteed. of the The onreader reason (see Figure DIN isA4that 1). (a different Note the shift substratethat despite in the frequency the than theresponse fact that resonator the resonators of thea reader), (when resonant of the tag element system were printed of the tag is on functionality on DIN A4 (a top of theThe guaranteed. different resonator reason of substrate the reader) is that than the the shiftisinmainly resonator of determined the frequency the reader), by the response system geometry functionality of such elements,is guaranteed. and this The reason iswhen is optimum that the shift in the of the geometries frequency response both elements are Appl. Sci. 2021, 11, 7832 4 of 12 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 µm) to a continuous line (drop spacing 20 µm) and then gradually separates into isolated drops (drop spacing over 50 µm). Therefore, the best drop spacing to achieve a uniform pattern is 20 µm since the drop dimension is 42 µm ± 2 µm (Figure 3a). Once the drop spacing is fixed, an assessment of the conductivity of the printed patterns in the function of the number of layers should be performed. Since the substrate used in this work is based on cellulose, the ink deposited onto it tends to be absorbed through it due to the capillarity. This fact may produce a decrease in the resolution compared with polymeric substrates because the pattern not only stands onto the surface, but also in the cellulose bulk. Hence, this configuration leads to a reduction in the conductivity as the printed pattern comprises both the non-conductive cellulose and the PEDOT:PSS. Therefore, we demonstrated the capability to obtain a conductive pattern onto cellulose without the need to seal and passivate the porosity with a primer. Figure 3b,c show optical images of inkjet-printed squares printed with one, two, and three layers using a drop spacing of 20 µm. As can be seen, the patterns present sharp edges even with 3 layers, meaning that there is no loss of resolution. Finally, Figure 3d shows a twenty-bit tag and a zoom of one resonator printed with one, two, and three layers of PEDOT:PSS. Van der Pauw measurements were carried out (Figure 4) for one, two, and three layers on paper in order to extract the sheet resistance. The study was conducted both with and without annealing the samples, since the paper is a temperature-sensitive substrate, although this step is used to remove the solvent. The thermal annealing treatment was carried out in the annealed samples by applying 110 ◦ C for 45 min in a conventional oven. A noticeable effect in all the cases was the higher sheet resistance in the range of 103 Ω/square compared with the samples printed on polymeric substrates 15–400 Ω/square, because of the penetration of the ink into the cellulose hampering the conductivity. In addition, the increment of the layers contributed to the reduction of the resistance for both annealed and non-annealed samples. The annealing process led to a resistivity sheet resistance of 1.2–1.3 × 103 Ω/square, whereas the sheet resistance of the non-annealed samples increased to 4.9–4.7 × 103 Ω/square, where the absorption of the solvent was attributed to the cellulose fibers. Despite the higher sheet resistance, the removal of the annealing process brings new opportunities for using cellulose-based substrates while simultaneously producing a substantial reduction in the total cost of the tags. Thus, in our study, the RFID tags were fabricated without annealing. Appl.Sci. Appl. Sci.2021, 11,x7832 2021,11, FOR PEER REVIEW 5 5ofof12 12 (a)Printed Figure3.3.(a) Figure Printedline linetopologies topologiesacross acrossaabroad broadrange rangeofofdrop dropspacing; spacing;(b) (b)inkjet‐printed inkjet-printedsquares squares printed printed with two, and with one, two, andthree threelayers; layers;(c)(c) zoom zoom of of thethe printed printed squares squares and and (d) 20‐bit (d) 20-bit tagaand tag and a zoom zoom of oneofresonator one resonator printed printed withtwo with one, one,and twothree and three layers.layers. Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 12 Appl. Sci. 2021, 11, 7832 6 of 12 5.4x103 Non-annealed 5.1x103 Annealed 4.8x103 Rsheet ( ) 4.5x103 1.6x103 1.4x103 1.2x103 1x103 1 Layer 2 Layers 3 Layers PEDOT:PSS printed Figure 4. Van der Pauw measurements as a function of one, two, and three PEDOT:PSS ink layers. Figure 4. Van der Pauw measurements as a function of one, two, and three PEDOT:PSS ink layers. 4. Experimental Tag Validation and Results In this section, 4. Experimental three different Tag Validation 20-bit tags with all bits set to ‘1’ were fabricated with and Results one, two, and three printed ink In this section, three different 20‐bit tags layers in order withtoallcompare bits set the effect to ‘1’ wereoffabricated the resonator withink one, two, and three printed ink layers in order to compare the effect of the resonator ink of conductivity on the measured envelope signal. The tags consisted of a linear chain resonant elements, conductivity identicalenvelope on the measured to the one of theThe signal. reader tagsline. In addition, consisted twochain of a linear trigger ofbits res‐set onant elements, identical to the one of the reader line. In addition, two trigger bits set tothe to ‘1’ were added at the beginning and at the end of the tag chain in order to know start and end of the reader system to acquire the data signal. The tags were fabricated by ‘1’ were added at the beginning and at the end of the tag chain in order to know the start inkjet printing using PEDOT:PSS on ordinary paper substrates (DIN A4). The measured and end of the reader system to acquire the data signal. The tags were fabricated by inkjet dielectric constant and loss tangent of the paper were εr = 3.11 and tanδ = 0.036 and the printing using PEDOT:PSS on ordinary paper substrates (DIN A4). The measured dielec‐ measurements were carried out by means of the resonant cavity Agilent 85072A. tric constant and loss tangent of the paper were εr = 3.11 and tanδ = 0.036 and the meas‐ The measured envelope functions of the fabricated tags are depicted in Figure 5. As urements were carried out by means of the resonant cavity Agilent 85072A. can be appreciated, the functional resonators (providing the logic state ‘1’) are detected The measured envelope functions of the fabricated tags are depicted in Figure 5. As by means of dips in the envelope functions, and tag reading provides the corresponding can be appreciated, the functional resonators (providing the logic state ‘1’) are detected by ID code. However, as the number of printed layers decreases (representing a reduction means of dips in the envelope functions, and tag reading provides the corresponding ID in the conductivity), the modulation index in the envelope function decreases, and this code. However, as the number of printed layers decreases (representing a reduction in the jeopardizes the detection of the ID code by means of post-processing stages. For example, conductivity), the modulation index in the envelope function decreases, and this jeopard‐ in the tag printed with one ink layer, it is difficult to obtain the ID code, especially when the izestag theisdetection read face of the ID down. On code the by meansby contrary, of printing post‐processing stages. two or three For example, layers, in the good modulation tagindex printed for both sides of the tag are obtained, and it suffices to detect the tag code ontag with one ink layer, it is difficult to obtain the ID code, especially when the both is read faces. face down. Note thatOn thethe contrary, different by printing signal two or offset levels werethree layers, as obtained good modulation a function of thein‐tag- dexreading for both sides of the orientation. tag are When the obtained, tags are read andface it suffices up, theto detect the envelope tag offsets signal code on areboth in the faces. same level (near to 1,5V). However, when the tags are read face down, they are in atag‐ Note that the different signal offset levels were obtained as a function of the lower reading level andorientation. differentWhenfor the the tagstags. three are read This face up, theis,envelope difference in part, duesignal offsets to the are in the ink conductivity same level (near to 1,5V). However, when and the air gap between the tag and the reader [57]. the tags are read face down, they are in a lower level and different Another for the three example tags. This of a 20-bit difference tag envelope is, in is signal part, due toin depicted the ink conductivity Figure 6. In this case, andonly the air twogap between layers were the tag and printed, andthe reader it was [57]. to detect the ID code clearly. Note that enough Another when a bit example of aa very is set to ‘0’, 20‐bitsmall tag envelope variationsignal of the is depicted envelope in Figure signal 6. In this is observed case, of because only two layers were printed, and it was enough to detect the ID the absence of a certain resonant element at their predefined position (the chain resonator code clearly. Note that is when a bit is set to ‘0’, a very small variation of the envelope signal detuned). With these results, the functionality of the proposed tags, implemented by means is observed because of theofabsence organic ofinks a certain resonanttwo and printing element layersat on their predefined ordinary paper position (the chain substrates, resonator is was demonstrated. detuned). With these results, the functionality of the proposed tags, implemented by means of organic inks and printing two layers on ordinary paper substrates, was demonstrated. The small difference between the face up and face down measured envelope func‐ Appl. Sci. 2021, 11, 7832 tions allows the addition of a cellulose layer on top of the printed resonators in order 7 ofto 12 hide them. This approach improves the security of the printed tags because after printing, the resonant elements are buried (sandwiched) in the paper substrate. Appl. Sci. 2021, 11, x FOR PEER REVIEWFigure 5. Measured envelope functions of the fabricated tags as a function of one two and 8three of 12ink layers printed. Figure 5. Measured envelope functions of the fabricated tags as a function of one two and three ink layers printed. Figure 6. Measured envelope functions of the fabricated tag printed with two ink layers. Figure 6. Measured envelope functions of the fabricated tag printed with two ink layers. The small difference between the face up and face down measured envelope functions allows the addition of a cellulose 5. Tag Re‐programmability layer onScaling‐Up and Industrial top of the printed resonators in order to hide them. This approach improves the security of the printed tags because after printing, the As mentioned before, the tags are linear chains of identical resonators, where each resonant elements are buried (sandwiched) in the paper substrate. resonator provides a bit of information when it is interrogated by the harmonic signal tuned to the resonance frequency. Such tags can be programmed either by cutting the resonant elements associated with the logic state ‘0’ (making them inoperative as shown in Figure 7a, or erased, by short‐circuiting the cut resonator through inkjet (thus adding conductive ink in order to set the corresponding bit to ‘1’, as shown in Figure 7b. The main Appl. Sci. 2021, 11, 7832 8 of 12 Figure 6. Measured envelope functions of the fabricated tag printed with two ink layers. 5.5.Tag Tag Re‐programmability Re-programmability and and Industrial Industrial Scaling‐Up Scaling-Up Asmentioned As mentionedbefore, before,thethe tags tags are are linear linear chains chains of of identical identical resonators, resonators, where where each each resonator provides resonator provides aa bit bit of of information information when when itit is is interrogated interrogated by by the the harmonic harmonic signal signal tuned to tuned to the the resonance resonance frequency. frequency. SuchSuch tags tags can can be be programmed programmed eithereither by by cutting cutting the the resonant elements associated with the logic state ‘0’ (making them resonant elements associated with the logic state ‘0’ (making them inoperative as showninoperative as shown inFigure in Figure7a,7a, or or erased, erased, by by short‐circuiting short-circuiting the the cut cut resonator resonator through through inkjet inkjet (thus (thus adding adding conductive ink in order to set the corresponding bit to ‘1’, as shown in Figure conductive ink in order to set the corresponding bit to ‘1’, as shown in Figure 7b. The main 7b. The main advantage of advantage of tag tag programming/erasing programming/erasing is is the the possibility possibilityof ofmass-fabrication mass‐fabricationofofall-identical all‐identi‐ cal tags, thus reducing fabrication costs. Once fabricated, the tags can be programmedatata tags, thus reducing fabrication costs. Once fabricated, the tags can be programmed alater laterstage, stage,and anderased erasedand and re-programmed re‐programmed as as many many times times asas needed. needed. However, However, multiple multiple tag programming/erasing cycles may give rise to tag degradation. In tag programming/erasing cycles may give rise to tag degradation. In typical applications,typical applications, tags should tags should be be programed programed only only once once in in order order toto provide provide aa unique unique IDID code code after after being being fabricated (multiple erasing and reprograming is not expected in fabricated (multiple erasing and reprograming is not expected in most applications). most applications). . Figure 7. (a) Identical resonators pre-programmed all to a logic ‘0’ and (b) re-connected resonators Figure 7. (a) Identical resonators pre‐programmed all to a logic ‘0’ and (b) re‐connected resonators (programmed to a logic ‘1’). (programmed to a logic ‘1’). A late-stage codification process can be implemented by additive inkjet printing (se- lective short-circuits) or laser ablation (physical destruction) procedures to be incorporated to the chipless RFID tag design and into the digital workflow standards already settled in the printing industry. For this purpose, Intense Pulse Light (IPL) post-processing of the printed structures on the millisecond time scale enable single line-scan functionalization of the required area, i.e., selective sintering of the light absorbing structures on the full print layout width, in an R2R compatible manufacturing process, thus bringing a whole different response to the interrogation signal. Watermarks, serial numbers, holograms, threads, encoding, specific molding, etc., are used to prevent copying and counterfeiting of numerous valuable documents, such as ID cards, banknotes, medical prescriptions, certificates, diplomas, etc., making the list of coding applications for chipless RFID paper enormous. For security paper applications, the proposed near-field chipless RFID system provides secure and low-cost encoders with unprecedented high data capacity (comparable to that of chipped tags), maintaining recyclability. Data storage resides on the printed tags, not in any further programming step. The tag design and the packaging graphics are printed simultaneously, only resulting in a slight increase in the cost of the package or tag. From the manufacturer’s side, the substrate has no visible change and no further adaption of the production process needs to be implemented. In addition, chipless RFID solutions are printed on the inside, or are hidden below different layers, inaccessible for intentional manipulation. Appl. Sci. 2021, 11, 7832 9 of 12 6. Conclusions In this paper, we have discussed fundamental aspects of the development of fully recyclable and organic chipless RFID technology mainly focused on the implementation and measurement of tags. These tags can be manufactured using printing processes, such as inkjet printing with organic conductive ink on conventional paper substrates. In addition, a time-domain chipless RFID approach, where the tags are read through near-field coupling (with sequential bit reading) by means of a harmonic interrogation signal, is reported. Validation examples have been discussed and reported, and we have shown that reasonably good results can also be obtained by printing only two ink layers, reducing the manufacturing and sintering time. It has been pointed out that this novel and unconventional system is of special interest in applications involving secure paper, where tag reading by proximity may represent an added value in terms of confidence. In addition, this is a real improvement for an eco-friendly world because the printing processes, such as flexography or inject printing techniques, using organic inks can achieve a lower environmental impact, and the tag unit cost is comparable to that of an optical barcode. This demonstrates the potential of chipless RFID organic technology, which should represent a very good choice for identification applications in the years to come. The adoption of chipless RFID technology opens a new path to low-cost fully printed chipless RFID solutions where the complexity migrates from the tag to the reader, making it possible to break the eurocent cost barrier. Author Contributions: C.H. and F.P. conceived the reported time-domain chipless RFID approach and designed the active part of the reader and the tags; M.M. developed the electronics necessary for tag reading, carried out the measurements, as well as, analyzed the data; C.M.-D. and R.E. carried out the validation experiments with the inks and printed the tags; M.M. and C.M.-D. wrote the paper; E.R. and F.M. reviewed, edited and supervised the work; L.T. supervised the work; E.R. and F.M conceived the work and they have been the directors of the research. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by MICIU-Spain (RTI2018-102070-B-C21), MICIU-Spain (RTC2019- 007226-7), Marie Skłodowska-Curie No 801342 (Tecniospring INDUSTRY), Agència per a la Com- petitivitat de l’Empresa de la Generalitat de Catalunya, by FEDER funds, MICINN-Spain (project PID2019-103904RB-I00), by Generalitat de Catalunya (project 2017SGR-1159), by Institució Catalana de Recerca i Estudis Avançats (who awarded Ferran Martín), and by ERDF funds. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Exclude this statement because do not report any data. Acknowledgments: The authors would like to thank our R&D partners: Fujifilm Dimatix, Ceradrop and DuPont Teijin for providing us with the research related consumables and their technical details. Conflicts of Interest: The authors declare no conflict of interest. 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