This document summarizes the development of a wearable RFID sensor for monitoring contractions and respiration. The sensor uses a knitted textile folded dipole antenna inductively coupled to an RFID microchip, avoiding soldering. Testing showed acceptable performance between 5-10mm from the body but optimal performance over 45mm away. Analysis of backscattered power from a pregnancy emulator showed agreement with commercial tocodynamometry for monitoring biometrics remotely and comfortably.

1. A Wearable RFID Sensor and Effects of Human Body Proximity
Damiano Patron1, Kahlil Gedin1, Timothy Kurzweg1, Adam Fontecchio1, G. Dion2, and Kapil R. Dandekar1
1 Electrical and Computer Engineering Department, 2 College of Media Art & Design, Drexel University, Philadelphia PA, USA
OVERVIEW
KNITTED RFID SENSOR DESIGN
HUMAN BODY LOADING EFFECTS
The aim of this project is to develop a wearable batteryless
wireless sensor, which replaces the current cumbersome wired
probes for contractions and respiration monitoring. We have
designed a wearable sensor made by a textile folded dipole
antenna and an inductively coupled RFID microchip.
The knitted tag antenna has been designed for use with a
novel inductively-coupled RFID microchip, which doesn’t
require any soldering process since the pads are inductively
coupled to the antenna metallization. The folded dipole topology
was chosen for matching with the microchip complex impedance
Zc= 25 – j 200 between 870 and 915 MHz.
IMPEDANCE MEASUREMENT RESULTS
The folded dipole topology is characterized by having balanced
input impedance, which doesn’t allow for conventional scattering
parameters (S-parameters) characterization using the unbalanced
ports of a vector network analyzer. However, the impedance of
the knitted antenna has been determined by the following
procedure:
 Input impedance extraction through 2-port fixture and S-
parameters measurements:
 Reflection coefficient S11 between Za (antenna) and Zc
(microchip):
A parametric simulation was run to observe the loading effects
at different heights d above the human stomach model. At 870
MHz, the S11 falls below the -6 dB threshold when the distance is
between 5 mm < d < 10 mm. However, the optimal S11 regime
under -10 dB is achieved when d > 45 mm.
ONGOING WORK
Biomedical data such as contractions and respirations are
monitored by processing the backscattered power (RSSI)
received by an RFID reader. Through a commercial RFID reader
and a pregnancy emulator, the analysis of the backscattered
power showed good agreement with
RFID Microchip
L = 120 cm
W = 10 cm
700 750 800 850 900 950 1000
0
50
100
150
200
Measured Input Impedance
Frequency (MHz)
Za
()
700 750 800 850 900 950 1000
-40
-30
-20
-10
0
Reflection Coefficient
Frequency (MHz)
|S11
|(dB)
𝑍 𝑎 = 𝑅 𝑎 + 𝑗 𝑋 𝑎 = 2 𝑍0
1 − 𝑆11
2
+ 𝑆21
2
− 2𝑆12
1 − 𝑆11
2 − 𝑆21
2
𝑆11 = 20𝑙𝑜𝑔10 1 −
4𝑅 𝑎 𝑅 𝑐
𝑍 𝑎 + 𝑍 𝑐
2
Xa = 196
Ra = 27
Radiation Pattern: (a) Simulated 3D radiation pattern. (b) Measured radiation pattern:
Azimuth (x-z plane) and Elevation (x-y) planes.
(a) (b)
HUMAN STOMACH MODEL
OMACH MODELHuman Stomach
Layers
εr Conductivity
(S/m)
Density
(Kg/m3)
Skin 41.4 0.87 1100
Fat 5.5 0.05 916
Muscle 56.9 1.00 1041
Intestines 55.0 1.05 1000
Loading effect on the radiation pattern: (a) When the antenna is kept very close to the
human body, the efficiency and the gain degrade significantly. (b) When the distance is
greater than d > 10 mm, the efficiency improves and the antenna radiates with positive gain.
(a) (b)
d= 5 mm d= 45 mm
Knitted antenna design: (a) 3D model and layout dimensions. b) Knitted prototype and
anechoic chamber setup for radiation pattern measurements.
(a) (b)
Remote 24/7 monitoring
Comfortable
The analysis was
extended to evaluate
the loading effects of
human body proximity.
the output of a commercial
tocodynamometer.