Inkjet-printed RFID antenna sensor for strain monitoring

A folded patch antenna has been previously proposed by the authors as a passive wireless sensor for strain and crack sensing. The electromagnetic resonance frequency of the antenna sensor varies correspondingly with physical deformation of the antenna. When the battery-free antenna sensor is bonded on a structural surface, strain and crack can be detected through convenient wireless interrogation. To further reduce sensor production cost, this research investigates inkjet-printing technique for fabricating the antenna sensor. Silver nanoparticles are inkjet-printed on a polyimide thin film to produce the antenna pattern. The printed silver layer replaces copper (as in previous fabrication of antenna sensor) to function as conductor on the antenna. For comparison, antenna sensors are fabricated with different numbers of inkjet-printed layers. To study strain sensing performance, printed antenna sensors are mounted on aluminum specimens for tensile testing. The strain sensing results are compared the previous antenna sensor with copper conductor. Both strain sensitivity and linearity of the printed antenna sensors are similar to previous copper antenna sensor. achieve wireless data interrogation (Balanis 1997). An off-the-shelf RFID chip, SL3ICS1002 made by NXP Semiconductors, is incorporated with the folded patch antenna. Enabled by the 1mm × 1mm RFID chip, frequency shift of the antenna sensor can be wirelessly modulated and interrogated by a reader. Previously, the antenna sensor is fabricated by milling machines or wet etching techniques. While providing accurate and reliable fabrication quality, these two methods entail expensive production cost (Vyas et al. 2011). To further reduce sensor production cost, this research investigates inkjet-printing techniques for fabricating folded patch antennas. Instead of using copper to conduct electromagnetic current, as in previous fabrication, a thin silver layer is inkjetprinted to replace copper for current conduction. Inkjet-printing technique has been widely adopted for printing RFID temperature sensor, gas sensor, antennas, and other radiofrequency electronics (Shaker et al. 2011; Vyas et al. 2011). Different inks can be used in printing, such as silver nanoparticles (Merilampi et al. 2011) and carbon nanotubes (CNTs) (Occhiuzzi et al. 2011). In this research, silver nanoparticle ink is investigated for inkjet-printing the conductive surface layer of the antenna sensor. To achieve better uniformity in the printed silver, the printing is performed on a flexible and ink-absorbent 5-mil Kapton polyimide film (instead of directly on Rogers RT/duroid® 5880). Bottom of the Kapton thin film is then bonded on a Rogers RT/duroid® 5880 substrate. Compared with previous copper antenna sensors, dimensions of the printed sensor are re-designed to consider the electromagnetic effect of adding the Kapton film. To validate strain sensing performance, the printed antenna sensor is mounted on an aluminum specimen for tensile testing. The strain sensing results of the printed antenna sensor are further compared with the previous copper antenna sensor. The paper is organized as follows. Section 2 reviews the strain sensing mechanism of the RFID antenna sensor. Section 3 first describes the design modification for the printed antenna sensor to accommodate the effect of the 5-mil Kapton HN thin film, and then presents strain sensing simulation results of the printed antenna sensor. Section 4 introduces fabrication procedures of printing, curing, and assembling the printed sensor. Section 5 discusses the tensile test results of the printed patch antenna. 2 STRAIN SENSING MECHANISM OF AN RFID ANTENNA SENSOR An RFID sensing system generally consists of an RFID reader and an RFID sensor. An RFID sensor contains a small antenna for transmitting and receiving electromagnetic wave, as well as an RFID chip for modulating the electromagnetic wave. The RFID adopted in this research is the SL3ICS1002 model manufactured by NXP Semiconductors. During operation, the RFID reader first emits an interrogation electromagnetic signal to the sensor. If the antenna sensor is within the interrogation range of the reader, some of interrogation power can be captured by the antenna sensor. Once the received power is higher than the turn-on power of the RFID chip, the chip is activated and sends a modulated signal back to the reader. In order to wirelessly activate the antenna sensor, the minimum amount of power transmitted by the reader is defined as the interrogation power threshold. A Tagformance Lite RFID reader (Voyantic Inc. 2011) is adopted for interrogation power threshold measurement. At each frequency point, the reader varies the interrogation power until the power is just enough to activate the RFID chip. After the reader sweeps through the entire frequency range of interest, the interrogation power threshold versus frequency curve can be obtained. The curve reaches its minimum point at antenna resonance frequency. When the antenna sensor deforms due to strain, the antenna resonance frequency changes accordingly. Therefore, strain sensing is achieved utilizing the relationship between antenna resonance frequency and strain. Details of the sensing mechanism and RFID interrogation can be found in Yi et al (2011). 3 PRINTED ANTENNA SENSOR DESIGN AND STRAIN SENSING SIMULATION RESULTS Because the Rogers RT/duroid®5880 substrate (31mil thickness) is hydrophobic, the substrate surface is not absorbent to silver nanoparticle ink and not suitable for directly printing upon. Therefore, prior to designing the printed antenna sensor, some printing trials are performed. Based upon the trials, Kapton HN thin film with 5-mil thickness is selected as the printing substrate to provide a more absorbent surface to silver ink, so that the printed silver ink layer has acceptable conductivity. The film has a tensile modulus of 370 ksi at room temperature, and provides elongation up to 82% at room temperature. However, because the 5-mil Kapton film is much thinner than the 31-mil Rogers substrate, antenna performance deteriorates if the Kapton film is used to completely replace the Rogers substrate. The performance deterioration includes decrease in both antenna gain and interrogation distance. Furthermore, the Kapton film has a higher relative permittivity (εr=3.5) compared with previous Rogers substrate (εr=2.2). The higher relative permittivity can also result in larger signal loss. Therefore, the Kapton film is not used to completely replace the Rogers substrate, but rather as the thin film to print upon. To complete the entire antenna sensor with comparable performance as before, the Kapton film printed with silver ink is laid on top of Rogers substrate. Fig. 1 shows that from top to bottom, the four layers of the antenna sensor are the printed silver ink, the 5-mil Kapton HN thin film, 31-mil Rogers RT/duroid®5880 substrate, and the copper ground plane. The manufacturer of the Rogers substrate provides ordering option that allows the substrate to be purchased with a copper layer attached on one side. Adding a 5-mil Kapton HN film into the original copper antenna sensor (Fig. 2a) changes electromagnetic behavior of the sensor. To continue using RFID interrogation, resonance frequency of the printed antenna sensor needs to maintain at around 915 MHz. The resonance frequency, f, of a folded patch antenna can be estimated according to the following equation (Balanis 1997):