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
gaetano.marrocco@uniroma2.it
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.
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[1]
[2]
[3]
203
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S. Milici, S.Amendola, A.Bianco and
d G. Marrocco, “Epidermal RFID
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