Inkjet-Printed Paper/Polymer-Based “Green” RFID and Wireless Sensor Nodes: The Final Step to Bridge Cognitive Intelligence, Nanotechnology and RF?

Manos M. Tentzeris, Anya Traille, Hoseon Lee, Amin Rida, Vasilis Lakafosis and Rushi Vyas School of ECE Georgia Tech

Atlanta, GA 30332-250 etentze@ece.gatech.edu

Abstract—In this talk, inkjet-printed flexible antennas, RF electronics and sensors fabricated on paper and other polymer (e.g. LCP) substrates are introduced as a system-level solution for ultra-low-cost mass production of UHF Radio Frequency Identification (RFID) Tags and RFID-enabled Wireless Sensor Nodes (WSN’s) in an approach that could be easily expanded to other microwave and wireless “cognition” apprlications. The talk will also touch up the state-of-the-art area of fully integrated wireless sensor modules on paper or LCP and demonstrate the 2D and the first ever 3D sensor integration in the form of module-on-paper, that could potentially set the foundation for the truly convergent wireless sensor ad-hoc networks of the future with enhanced cognitive intelligence and “rugged” packaging. Various examples encompassing temperature sensors, carbon-nanotube-enabled inkjet printed gas sensors and crack detection sensors will be covered in detail.

I. INTRODUCTION The demand for low cost, robust, flexible, reliable, low-power consumption and durable wireless modules such as RFID-enabled sensor nodes is driven by several applications, such as logistics, Aero-ID, anti-counterfeiting, supply-chain monitoring, space, healthcare, pharmaceutical, military and is regarded as one of the most important methods for realizing ubiquitous ad-hoc networks. In general, RFID’s could constitute a very low cost integration platform of multifunctional capabilities while allowing a low-power wireless communication implementation. Paper material is one of the candidates that could potentially cause a break through in the electronics world for its ultra low cost and environmental friendly characteristics. This could realize a truly “ubiquitous computing” network. From a manufacturing perspective, paper can go large reel to reel processing and so is a perfect candidate for bulk production especially when accompanied by a fast direct write methodology such as inkjet printing. This could be done since paper has a low surface profile and with the appropriate coating can host conductive paste on top of its surface which

enables modules such as: antennas, IC, memory, batteries and/or sensors to be easily embedded in/on paper. Paper is an appropriate organic-based substrate for UHF and RF applications for several reasons. Its wide availability, the high demand and the mass production make it the cheapest material ever made. Being environmentally friendly along with the capability of fast printing processes such as direct write methodologies of\ conductors make it very attractive for manufacturing. In addition, paper can be made hydrophobic and/or fire-retardant by adding certain textiles to it. While the electrical characterization of dielectric constant and loss tangent of paper initially up to 2.4 GHz [1] and lately up to 15 GHz, it had become possible to start developing 2D and 3D sensing modules encompassing wide frequency ranges. Inkjet-printing is a direct-write technology by which the design pattern is transferred directly to the substrate, and there is no requirement of masks contrary to the traditional etching technique which is widely used [2]. In addition, unlike etching which is a subtractive method by removing unwanted metal from the substrate surface and which also uses chemicals such as the etchants throughout the fabrication process, inkjet printing jets the single ink droplet from the nozzle to the desired position, therefore no waste is created, resulting in an economical fabrication solution. This aspect, together with the fact that the chemicals necessary for etching are eliminated, makes this approach environmentally friendly also. Curing the conductive particles such as nano-silver (used in this paper) has to occur before any operation of the ink as a good conductor. Curing at temperature high temperatures is desirable; however, other methods such as UV or photonic curing may be used, which takes up only few seconds. The inkjet printing on-paper approach is very repeatable, allows for features down to 20um and can be easily utilized for other passive structures, such as filters, baluns in single or multilayer (multi-sheet) configurations [2].

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II. WIRELESS SENSOR MODULES ON PAPER Integrating sensors in the RFID tags renders the whole

system capable of not only tracking, but also providing real-time cognition of aspects of its status or its environment. The ultimate goal is to create an easily deployable and “rugged” intelligent network of RFID-enabled sensors A general system level design for the wireless transmitters can be seen in Fig. 1. At the heart of the system is an integrated 8-bit Microcontroller Unit (MCU), which uses a high-performance Reduced Instruction Set Computing (RISC) architecture and is programmed using Assembly code.

Software-wise, it is designed to operate in three different modes, namely “UNMOD”, “SENSE” and “SLEEP”, which can be selected with a push button. In "UNMOD" mode, the MCU is programmed to turn on the communication module, and have it send out an unmodulated signal at the design frequency of 904.4 MHz. In this mode, during the full unmodulated signal transmission, the power consumption is measured to be 36 mWatts, which enables the RFID tag to operate for a maximum period of 3.75 hours with a 3V

battery capable of supplying 45 mA-hours [3]. In "SENSE" mode, the MCU reads temperature values from an external temperature sensor and uses the communication module to send out this information using an, initially Amplitude Shift Keying (ASK) and later on Frequency Shift Keying (FSK) modulated, signal. As also explained later, the ASK mode was chosen for easier and more reliable wireless link

measurements and was replaced by the FSK modulation for the communication with the WSN MICA2 mote's TI CC1000 transceiver, as described in section VI. Regarding the "SLEEP" mode, the MCU is programmed to disable the communication module and go into sleep mode, an extremely low power state (1.8 µWatts), in order to conserve power [4,5].

The data transmission is carried out at the unlicensed Industrial, Scientific and Medical (ISM) frequency band at 904.4 MHz. This transmit frequency is generated with the help of an external crystal oscillator fed into the communication module of the IC, shown in Fig.3 , which comprises of a Phase Lock Loop (PLL) module with a PA at the output of the wireless transmitter. The communication module, when enabled by the MCU, takes in the input or reference frequency generated by the RF crystal oscillator

and generates an output signal locked in at 32 times the reference frequency. The MCU is also programmed to control the wireless communication by modulating the PA in the integrated communication module at calculated intervals, thereby giving an amplitude shift keyed (ASK) modulated signal at the output of the PA with the required data rate [3]. The modulation by the MCU is carried out in the same sequence as the bit encoded sensor data that is sampled and averaged during the previously described “SENSE” mode. The output data rate is generated by using the internal clock of the MCU to toggle the RF PA.

The transmission range of the RFID tag is directly proportional to the amount of power transferred from the PA to the transmitter front end to the antenna. Any impedance mismatch between the two can lead to the internal reflection of a part of the power intended to radiate out of the antenna, thereby minimizing the power transmitted. For this reason, the determination of the optimum load impedance looking out of the PA in the wireless front-end, so that the antenna can be designed to match to this exact load is of vital importance. This PA characterization is achieved with a load pull analysis performed at 904.4 MHz [8]. The optimum load impedance ZOPT looking out of the PA at close to the transmit frequency

Fig. 2. 3D normalized radiation pattern of the proposed antenna

Fig. 1. System level diagram of the wireless sensor modules

Fig. 3. Inkjet-Printed Wireless Sensor transmitter prototype on paper

substrate.

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of 904.4 MHz after accounting for bias circuit effects was determined to be 36.95-j71.77 Ω .

The presented wireless sensor module prototype using a dipole antenna was printed on a 2-D (single layer) paper module. The overall dimensions of the structure are 9.5 by 6 cm and is shown in Fig. 3.

The antenna and the circuit layout were printed and cured on paper using silver ink and the complete wireless sensor system comprising the TSSOP packaged integrated circuit (IC), including the MCU and the transmitter, its discrete passive components including a crystal oscillator, the TC1047 temperature sensor, and a Li-ion cell for “stand-alone” autonomous operation were assembled on it.

To ensure maximum conductivity and antenna efficiency, the entire circuit was printed over with 12 layers of silver ink resulting in a conductor thickness of 12 microns [9].

Next, the wireless sensor modules were assembled on the printed circuit. The assembly process on silver pads proved to be the most challenging aspect in the design process. Given the low temperature tolerance of paper, the electronic components used, and the relative weaker adhesion of printed silver pads on paper, soldering had to be ruled out. Multiple assembly methods were experimented in order to find a reliable alternative for mounting components, which include silver epoxies and conductive tapes. The entire assembly process had to be carried out in a multi-step process [8]. This was because the conductivity of the silver epoxy had a direct co-relation to its curing time and temperature, which contradicted with the limited temperature tolerance of the components used in the wireless transmitter.

The impact of this module is by its ease in extending to a 3D multilayer paper-on-paper RFID/Sensor module by laminating photo-paper sheets. This is expected to reduce the cost of the sensor nodes significantly and eventually make the ubiquitous computing network a possible reality with a

convergent ability to communicate, sense, and even process information.

In order to verify the correct operation of the diploe-based wireless sensor module, it was triggered to operate in the “SENSE” mode. The transmitted sensor data have shown go

good agreement with the actual measurements carried out with a digital infrared thermometer, whose accuracy was ±2.5C.

III. INTERFACING WITH WSN MOTES A centralized and efficient way of gathering, storing and

processing huge amount of data by RFID enabled sensors becomes crucial. In particular, the objective is to be able to "read" the RFID tags not only over long RF ranges but even from the other side of the planet and to render this gathering automated without involving the human in the loop at a low cost.

The solution to this problem can be the deployment of Wireless Sensor Networks (WSNs) in between the RFID tags and the Internet, replacing existing RFID readers which are not only much larger in size than typical WSN motes but also two orders of magnitude more expensive. The bridging of the world of the RFIDs with that of WSNs brings in the spotlight the new area of “ubiquitous RFID networks”, which can provide truly cognitive intelligence over extended physical distances on top of very low-cost and mature wireless infrastructures. Within the scope of this paper, the WSN motes are not anymore the lower level network

devices; their primary task is not as data generators, sensing the world, but as data routers relaying the RFID sensed traffic through wireless multi-hop links to one or more sinks. Communication between the communication module of the aforementioned paper-based RFID tag and the commercial TI CC1000 transceiver-based Crossbow’s MICA2 WSN mote [10,11] was achieved for the first time using a very simple but power efficient communication protocol.

Fig. 5. ASK modulated temperature sensor data captured by the RTSA at room temperature (Power vs. Time).

Fig. 6. An example of a WSN topology

Fig. 4. Working prototype of Solar Powered Battery-less Tag

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ACKNOWLEDGMENT The authors would like to acknowledge the support of

NSF-ECS and IFC-ERC.

REFERENCES [1] EPC Global, Frequency Regulations, Available:

http://www.epcglobalinc.org/tech/freq_reg?, April 2010 [2] S.Y.Y. Leung, and D.C.C. Lam, “Performance of Printed Polymer-

based RFID Antenna on Curvilinear Surface” in IEEE Transactions on Electronics Packaging Manufacturing, vol. 30, issue 3, 2007, pp. 200-205

[3] Panasonic Magnesium Lithium Coin Batteries Specifications. Panasonic, 2005.

[4] R. Vyas, A. Rida, L. Yang, M.Tentzeris, “Design and Development of a Novel Paper-based Inkjet-Printed RFID-Enabled UHF (433.9 MHz) Sensor Node”, IEEE Asia Pacific Microwave Conference, pp 1-4, Dec 2007.

[5] J. Peatman, Embedded Design with the PIC18F452 Microcontroller. Upper Saddle River, NJ: Pearson, 2003, pp 51-68 & 116-131.

[6] R. Vyas, A. Rida, L. Yang, M.M Tentzeris, “Design, Integration and Characterization of a Novel Paper Based Wireless Sensor Module”, IEEE International Microwave Symposium, June, 2008.

[7] Dimatix datasheet http://www.dimatix.com/files/DMP-2831-Datasheet.pdf, April 2010

[8] A. Rida, L.Yang and M.M.Tentzeris, RFID Enabled Sensor Design and Applications, Boston, MA: Artech House, 2010

[9] M. Lessard, L. Nifterik, M. Masse, J. Penneau, R. Grob, R, “Thermal aging study of insulating papers used in power transformers,” Electrical Insulation and Dielectric Phenomena 1996, IEEE Annual Report of the Conference on, Volume 2, pp. 854 – 859, 1996.

[10] R. Vyas, V. Lakafosis, Z. Konstas, and M.M. Tentzeris, "Design and Characterization of a Novel Battery-less, Solar Powered Wireless Tag for Enhanced-Range Remote Tracking Applications", European Microwave Conference, Sep. 2009

[11] "CC1000 Single Chip Very Low Power RF Transceiver, Texas Instruments", 2008, [Online]. Available: http://focus.ti.com/lit/ds/symlink/cc1000.pdf

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