On-Skin Tunable RFID loop Tag for Epidermal Applications
Transcription
On-Skin Tunable RFID loop Tag for Epidermal Applications
On-Skin Tu unable RFID loop Tag for Epid dermal Applications S. Amendola, S. Milici and G. Marrrocco C. Occhiuzzzi University of Roma Tor Vergata Roma, Italy [email protected] RADIO6ENSE S.r.l Roma, Italy occhiuzzi@radio6en nse.com Abstract—The performance of dual-loop UHF-RFID epidermal antennas for application to healthcare and perssonal wellness are here investigated against the variability of the placement over the human body and of the user’s body mass. An on-body retuning mechanism is hence introduced and experimented. Finally two kin temperature applications are described for the on-sk monitoring and for the detection of fluids exch hange (sweat loss, drugs release) at the skin interface. I. INTRODUCTION In the last few years the fertile researcch on Epidermal Electronics [1] is paving the way to aan entirely new opportunities for personal wellness and healthhcare throughout the development of electronic devices cablle to seamlessly integrate with the epidermis and unobttrusively collect physiological parameters like pressure, bloood glucose level, hydration level and temperature. So far, m many efforts have been devoted to the design of multifuncttional conformal systems consisting of ultrathin and flexible ellectrodes, sensors and communication units, with particular foccus on the hightech fabrication processes and on thee synthesis of biocompatible functionalized materials with enhanced sensor/actuator potentialities. The real bbottleneck, still preventing a widespreading diffusion of theese skin-mounted devices, is the need to provide the poweer supply to the embedded electronics components and a real ease of interrogation by a remote scanner thus avoidinng the limitations of near contacting links. The RFID technology in the UHF bandd (860-940MHz) could be the natural candidate to overcom me above issues thanks to the capabilities of the passive taggs to be wireless powered-up and interrogated at a distance of some meters. However, conventional layouts of wearable anntennas provided with decoupling multi-layer spacers or grounnd planes are not suitable for epidermal applications that insttead demand for sensors to be placed in direct touch with thee skin and not to interfere with the local metabolism of epiderm mis. Therefore the behavior of epidermal-like tags in the UHF baand still deserves to be investigated in full details and hence masstered. This contribution examines the electrom magnetic response of a reference dual-loop tag that is fabriicated onto thin 978-1-4799-7815-1/15/$31.00 ©2015 IEEE Fig. 1. a) Layout of the epidermal tag g. Size [mm]: L1 = L2 = 50, w = 2, d = 0.5, a = 26, b = 10. b) Prottotype of Epidermal RFID tag transferred over an arm. biocompatible membranes and is suitable to a comfortable application over body curvature. Bo oth numerical modeling and extensive experimentations are performed at the purpose to y of tag performance with highlight the significant variability the placement over different body districts d and with the user’s body mass. An on-body tuning mecchanism is hence introduced and experimented to enable the use of a same tag layout for all the UHF RFID bands and for several placement loci over the body. Finally some pioneering seensing applications of this epidermal tag to the wireless measurement of body temperature and to wound care are here h reviewed. II. DUAL-LOOP TAG WITH ON-SKIN TUNABILITY The geometry of the considered epidermal-like e tag (Fig.1a) consists of one-wavelength rectang gular-loop fed by a smaller one whose shape is properly sizeed in order to match the impedance of the RFID IC (here th he NXP–G2X). This layout has been already proposed in [1]] as wearable tag for the known superior radiation performaance over dipoles close to lossy materials. However, the ep pidermal-like tag is now insulated from the human skin by only a thin silicone 2 =0.005 S/m) so that it membrane (thickness 600 m, = 2.5, is forced to work at very close touch with the skin. A maximum realized gain Gτ=-13 dB B was estimated at 940 MHz by numerical simulations over a simplified multilayered human phantom. 202 AP-S 2015 Fig. 2. Example of on-skin retuning from US to EU band of the epidermal tag placed onto the arm of the female vvolunteer. Unlike grounded wearable tags, the Gτ oof the epidermal antenna and, accordingly, its reading rannge, exhibits a substantial sensitivity to the placement oveer different body districts as well as on the user’s body mass: Gτ varies from 17dBi over stern to -9dBi over limbs and thhe corresponding read distance are roughly 1m and 2m, respecttively. In order to mitigate this variability it is hence desired to have a postfabrication retuning mechanism. The workingg principle of the here introduced method implies the perturbatiion of the surface current paths along the inner perimeter of thee smaller loop at the purpose to modify its equivalent selff-impedance and accordingly the mutual inductance between the two coupled loops [2]. As a result, the input impedance of the epidermal tag will become easily controllable. To practtically implement this solution, the segment of the smaller loop that is closest to the external loop is split into six equal-size ppre-cut strips that can be selectively removed at the purpose to sshift the matched frequency in a controllable manner. Fig.2 proovides a realistic example of how to easily readapt to EU frequencies an epidermal tag that was initially designed for aapplication in the US band. At this purpose three strips were rremoved after the placement over the volunteer’s arm. Fig. 3. a) Body Temperature profile collected c from epidermal tag deposited over PCL synthetic skin durring cycling. b) RF Intelligent Plaster on PVA/XG skin: absorbing sensing curves at 860MHz. B. Hydrogel-based RF Plaster Hydrogel dressings are being exttensively used for wound treatment thanks to their capabilitiees to absorb secretions from exudating wounds, to cool the infe fected areas, to recover the dehydrated tissues and release drugs (e.g. antibacterial agents). A hydrogel thin membrane made of polyvinyl alcohol/xyloglucan (PVA/XG) waas here used as sensing substrate of the epidermal tag. The T RF properties of the membrane are expected to significantly vary during the absorbing/swelling processes and consequently to sensibly impact on the matching and radiatiion performance of the tag. Standard UHF-RFID metrics such as the Gτ and the AID are suitable indicators to monitor thee evolution of the wound healing process. A preliminary experiment e considered the plaster tag placed onto a sponge floating on a saline water solution in order to mimic the fluid absorption process. The curves of and at 860 MH Hz vs. time appear quite linearly monotonic (Fig.3) with 8d dB and 3dB sensing range, respectively, from which an overaall 12% of fluid absorption can be roughly estimated. MAL TAG III. SENSING APPLICATIONS OF EPIDERM A. Wireless epidermal thermometer A prototype of the epidermal RFID tag was deposited over a Poli( -caprolactone)(PCL) synthetic skin (thhickness 500µm) whose fibrous structure was especially conceeived to preserve the natural transpiration and hence the local teemperature of the skin. The tag is equipped with the EM44325 RFID die integrating a digital temperature sensor ppowered by the interrogation signal. This wireless epidermal tthermometer was experimented in real conditions for the measurement of body temperature under physical stress. Fig.3 showss the temperature profile as it was sampled by the tag attached oonto the abdomen of a volunteer during cycling. The collected temperature, readable up to 1m, appears slightly swinginng accordingly to thermoregulation mechanisms and it is in substantial agreement with a reference thermocouple. REFERENCES [1] [2] [3] 203 Yeo, W.H., Kim, Y.S., et al. “Multtifunctional epidermal electronics printed directly onto the skin”, Advanced Materials, vol.25,, May 2013. Tsai, Min-Chuan, et al. “Inductively-ccoupled loop antenna de- sign for UHF RFID on-body applications”, Progress In Electromagnetics Research, Vol. 143, 315-330, 2013. S. Milici, S.Amendola, A.Bianco and d G. Marrocco, “Epidermal RFID Passive Sensor for Body Temperaature Measurements”, RFID-TA Conference, Sept.2014, Tampere (Finlland).