A new disposable radio-frequency identification (RFID) sensor for detecting
oxygen in packages with a protective atmosphere is presented. For safety
reasons and system costs in consumer packages, no battery or energy
harvesting devices can be used. Each part of a package, especially in food
packaging, must be completely safe even if it is swallowed. Several materials
have been investigated that safely react with oxygen and thus change
electrical parameters without the need of an additional energy supply. In
particular
linseed oil was tested, because it is known to react in oxygen-containing
atmosphere from liquid to solid. Linseed oil is used not only as food but
also as a key part in ecological paint coatings. A significant relative
change of capacity was observed during linseed oil drying, which results in
The oxygen sensor is coupled with an RFID front end with an internal charge time measurement unit for capacity determination. The combination of sensor element, sensitive material and RFID allows for biocompatible and save systems that indicate the presence of oxygen within a package.
Metal oxide gas sensors are widely used for oxygen detection. Semiconducting
metal oxide surfaces such as titanium oxide, zinc oxide and others show an
oxygen-dependent conductivity (Kreisl, 2006). Electrochemical oxygen sensors
for measuring liquids and gases are generally based on the amperometric
principle, in which the current of a galvanic cell can be measured. One kind
of amperometric oxygen sensor is the Clark electrode with liquid
electrolyte, which is the most often used oxygen sensor at room temperature.
In this case, the oxygen diffuses through a membrane into the liquid
electrolyte and generates a current (Clark and Lyons, 1962; Otto, 2011).
Furthermore, amperometric sensors with solid electrolyte, which are based on
doped zirconia as an ion-conduction material between two electrodes, are
employed as well (Tränkler and Obermeier, 1998; Kamp, 2003).
Paramagnetic sensors require a heating wire and a permanent magnet. Inside
the sensor, gas circulations occur through heating the gas molecules that
cool down on the outer wall. As oxygen is paramagnetic it will be
accelerated through magnetic repulsion to the wire and cools it at increased
rate. The changing heater resistivity can be measured (Reichl, 1989). All
these methods are suitable for continuous measurements of oxygen
concentrations. Furthermore, those solutions usually require thermal
activation energy (Xu et al., 2000) or an electrical potential difference
that complicates the usage in self-sufficient systems. In paramagnetic
sensors, temperatures of about 300
Additionally, those systems are complex, resulting in higher costs. These facts do not allow for the usage of sensing elements in packaging of mass products.
Due to cost and environmental constraints, sensing in end consumer packaging excludes energy sources such as batteries or energy harvesting systems. Additionally, hazardous substances could find a way into the packaged goods when those systems are damaged, e.g. during opening of the package.
Low-cost sensors for food application should be as simple as possible and preferentially set up from packaging or food ingredients. The usage of biocompatible substances is preferred. To read out the status, a wireless transmission via radio-frequency identification (RFID) is a suitable way for checking even larger quantities of packages at once. At least a simple identification of leaking packages is possible. Ideally the sensing should be irreversible at a defined oxygen concentration threshold.
To realise self-sufficient, autonomous sensor elements, sensitive materials with low activation energy are required. Some organic materials show a change of conductivity at room temperature. Polyacetylene, polypropylene and polyphenylen sulfide are listed as important representatives. Most of them are assumed to be toxic or lead to irritation of respiratory tracts. The acceptability with regard to food packages is not without limitations (Sigma-Aldrich, 2013; de Moraes Porto, 2012).
But also unsaturated fatty acids are sensitive to reactions with oxygen. In nature many variations and grades of unsaturation exist, which influence the reaction speed at oxygen contact. Some of them are used not only in food but also for applications such as paints and lacquers, sometimes for centuries.
Fatty acids are aliphatic monocarboxylic acids that include one carboxyl
group (RC(
The reaction with oxygen was analysed in a variety of publications. The following description is, for example, given in Kuang Chow (2007):
Electrode configuration for measuring capacity and
resistivity including the ability to heat the active area (left: outer
conductor (1) with a structure diameter
(
RH represents multiple unsaturated fatty acid molecules, which lead to a
creation of radicals R
For fast reactions, the percentage of multiple unsaturated radical molecules is essential. Dubois et al. (2008) shows some natural oils divided in three groups of fatty acids. Linseed oil shows a high percentage of multiple unsaturated bindings, which is an indication of short reaction times. Furthermore, after the oxidative polymerisation process, linseed oil approaches a stable condition by forming long-chain molecules. This is contrary to technical oils, which decompose in a reverse direction. During the oxidation, first the volume of linseed oil increases and finally decreases when the polymerisation starts. The reaction time depends on a multitude of ambient conditions such as humidity, temperature and oxygen concentration.
The sensor electrodes should be designed for high sensitivity. For this reason, an optimised ratio of large electrode area and small gap size should be obtained for an almost simple capacity measurement. In order to achieve reliable results, structures for sensing and referencing and the possibility for heating the structures to a reference temperature are included.
Structure sizes and simulated stray field height of the capacitor.
The designed interdigital structure is shown in Fig. 1, which has an aspect
ratio from circuit path width (
This design is able to measure capacity as well as resistivity values in a three-wire configuration. The outer conductor (1) can be used as a heating element, too. This allows for reaching stable and homogenous temperature distribution over the whole structure, if required. Both symmetrical inner conductors can be used for measuring (2) and referencing (3) for the electrical characterisation. The heating and measuring elements are manufactured from a single metallisation layer and allow, subsequently, for a simple fabrication process.
Cross sectional view and fabricated sensors with a diameter of 2.2 mm on bulk silicon (left) and on an AlN membrane (right).
In all, < 100 > silicon wafers (1–20
Furthermore, sensing devices can be fabricated on membranes by backside deep reactive ion etching (DRIE). This allows for low energy consumption if sensitive materials with higher activation energy require heating. Figure 2 shows devices with and without a membrane.
The initial resistivity and capacity of the conductors were characterized.
In Fig. 3, the capacity for different layouts listed in Table 1 at different
frequencies is shown. The ambient gas during this process was nitrogen with
a relative dielectric constant
Characterisation of capacity at selected structure
diameters (
The sensitive material is applied onto the sensing structure. The permittivity increases when the fatty acids/oils are introduced. In literature, values between 3.2 and 3.5 are reported for linseed oil. The resulting capacity splits up into the substrate capacity and effect due to the sensitive material. The sensitive material should preferentially cover the volume of the capacitive stray field to achieve reliable changes due to the oxidation. Therefore, COMSOL Multiphysics simulations were performed to determinate the required volume and thickness of the sensitive layer (Fig. 4).
Simplified stray field model of structure with 2.2 mm
diameter for simulating the area of high electrical fields (in this case
about 50
Table 1 shows simulated stray field heights for different layouts that have to be coated with sensitive material. The dispensing of the oils can be done in ambient air and the achieved capacity is measured. The delay before electrical parameters start to drift depending on the fatty acids and it is usually long enough (more than 10 min) to place the sensor into the measuring set-up. Linseed oil is a liquid. In an inert atmosphere, the capacity does not change and it stays liquid. If oxygen is present the unsaturated fatty acids start a reaction and consume oxygen molecules. The permittivity and thus the measured capacity changes during the reaction and equals a stable state after the polymerisation is finished. Figure 5 shows linseed oil before and after the oxidative polymerisation process on a sensor device.
Sensor (structure diameter
Figure 6 shows the changes of capacity of a structure with 3 mm diameter at ambient conditions (21 % oxygen concentration), 293 K and a relative humidity of 50 % in a frequency range between 100 Hz and 100 kHz. The capacity was determined without, with liquid and with solidified linseed oil in steady state. It can clearly be seen, that the solidified oil has a significantly lower dielectric constant as compared to the liquid state. Due to cross-linking of the fatty acids the reduction of the amount of polar groups causes a reduced dielectric constant.
Capacity of a structure with a diameter
(
Further experiments have been performed at a quasi-static state as the measurement principle of the RFID interface uses a similar method. Thus, a voltage ramp is used and the charging current is measured once an hour.
It was confirmed that the reaction speed depends on the humidity and concentration. Therefore, a set-up that allows for varying the concentration of oxygen, humidity and temperature in a measuring chamber is needed to achieve defined conditions. The concentration of oxygen is adjusted by using mass flow controllers for pure nitrogen and synthetic air. Humidity is adjusted by using a bubbler and a humidity sensor for control. Additionally, the chamber can be heated. The oxygen concentration is measured by using the oxygen sensor Mettler Toledo InPro 6800 G. Furthermore, sensors for pressure and temperature are included and allow for controlling the conditions inside the measurement chamber. Figure 7 shows a simplified scheme of the measurement set-up. Experiments are driven by a LabVIEW program, which allows for the setting and saving of the data. Sensors under test are assembled on a carrier module that connects sensor devices with a reusable RFID system.
Simplified diagram of the test station with magnetic valves (MV) for flow path selection, mass flow controller (MFC) and sensor element with connected read-out devices via RFID and quasi-static capacitive voltage metre (QSCV).
In Fig. 8, measured capacity vs. time at oxygen concentrations of 1, 10.25 and 20.5 % are shown at 293 K and 0 % relative humidity. In a second study, the influence of the moisture at constant oxygen concentration is presented (20.5 % oxygen concentration, 293 K). High humidity causes, as expected, an increase of capacity due to the high dielectric constant of water molecules. Due to polymerisation the influence of water decreases and results in a significant decrease of capacity.
Depending on the concentration of oxygen and moisture, the oxidative polymerisation requires a distinct time. Low oxygen concentrations as well as high humidity slow down the reaction.
Relative change of capacity by varying the humidity (
The reaction can be forced by a higher surface-to-volume ratio as achieved
by spin coating or by using materials with polyunsaturated fatty acids. Also
docosahexaenoic acid ethyl ester (DHA-EE), arachidonic acid and linoleic
acid that have 6, 4 and 2 double bindings, respectively, have been tested. These are
purified ingredients of linseed and tuna oil. Their reaction rate is more
reproducible than that of the natural linseed oil with naturally varying
contents of fatty acids. Figure 9 illustrates the relative change of capacity
in ambient air (21 % oxygen, 293 K, 50 % relative humidity).
Polyunsaturated fatty acids such as DHA-EE show a faster reaction as
compared to linoleic acid. A volume of about 0.5
Time-dependent changes of capacity through the oxidation of fatty acids with various unsaturation ions (linoleic acid with 2, arachidonic acid with four and DHA-EE with six unsaturated bindings).
A suitable interface between the sensor element and RFID is required for a stable energy supply and data communication. Theoretical investigations for the power transmission have been performed. Starting point was the equivalent circuit and the complex equation approach in Finkenzeller (2006). The transmitter is based on a resonant antenna circuit and a resistive load. The sensor element consists of a resonator connected to the sensor, which represents the load. The equivalent circuit is shown in Fig. 10.
The oscillator circuits of the RFID reader and the
transponder with connected sensor element described with the load resistance
The transfer function between the output voltage
The oxygen sensor is connected to a commercial RFID front end from microsensys GmbH. For the sensor transponder, an interface working at 13.56 MHz was developed. Because of the required power for the transducer and the controller, a high-frequency RFID interface is preferred, which was built according to the ISO14443 type B standard. Therefore, the figure of merit of reader circuit and transponder was adjusted for an optimal bandwidth by decreasing the figure of merit and maximum field strength. The resonance frequency of both oscillating circuits is optimized for the ability to connect other reading systems from microsensys GmbH and for a wide functional range of the system.
Here, the so-called “M30-Head” was chosen and special commands for the transponder were integrated. The protocol is based on the iID® 3000Pro protocol and complements this in order to achieve more flexibility. The sensor adaption is done by the transponder controller. Figure 11 shows the developed RFID transponder for resistive and capacitive read-out and the used M30-Head.
RFID-evaluation boards with front end controller for resistive and capacitive readout (left) and the used M30-Head for connection via USB to a processing unit (right).
The transponder consists of a commercial RFID front end iID-L® with voltage output, a microcontroller with integrated analogue–digital converter, and a unit to measure capacities. As the interface between front end and controller, a serial bus interface for data exchange is used. The output of the front end supplies a current of 2 mA for the sensor, e.g. for heating. For measuring the signal of the sensor, a high resistance input is used to analyse an analogue voltage as for measuring capacities. The design of the electronic compounds is optimised for a capacitance range between 50 and 90 pF, based on the measurements shown before. Figure 12 shows the block diagram of the used transponder.
For measuring capacitive changes, two approaches can be used:
A capacity-to-digital converter. Actually, this type is not available for
this low voltage and low power application. Charge time measurement unit (CTMU), which is already available in the
microcontroller. Thereby, a constant, adjustable current is injected for a
defined time and the potential is read out as a measure of the capacitance.
Block diagram of the transponder with integrated units for signal analysis and connections for the sensor element.
The measurement cycle is as follows: the capacitor is discharged before each
measurement. The connected capacity
Assuming a starting voltage
For final calculation of the capacitance
The capacitance
The sensor was connected to the RFID transponder. First, the value without
sensitive layer was read out. The application of the material causes changes
in the capacity. Thus, the value of the AD converter decreases. During the
reaction time, the ADC value increases. After polymerisation it nearly
reaches the value without sensitive material. The tests were performed at
different sensor sizes. Larger sensors show higher changes and the
evaluation is more accurate. By using Eq. (7), the ADC values can be
converted into capacitive values with the typical voltage
Furthermore, the behaviour of linseed oil over time was recorded. After 5 h, the RFID interface shows significant changes. After this time, damages of the package can be detected. By using other fatty acids, a much faster response is possible between the incidence of oxygen and measurable changes. The measured results of linseed oil are shown in Fig. 13.
Measured capacity and corresponding ADC values from RFID transponder for a sensor with linseed oil (293 K, 0 % rel. humidity) for a structure diameter of 3 mm.
Hence this demonstrates the applicability of these kind of sensors in combination with an RFID evaluation unit to observe the increase of oxygen in a protective gas atmosphere.
A new system concept of self-sufficient oxygen sensors is demonstrated, which records oxygen contamination without external energy. The intended application of the sensor is a threshold statement if unwanted oxygen access has occurred to the atmosphere under test. This is of interest for pharmaceuticals, for food under protective atmosphere and for industrial products such as oxygen-sensitive chemicals.
Therefore, materials which are safely reacting with oxygen were analysed. Unsaturated fatty acids, for example in linseed oil, polymerize in oxygen-containing atmospheres and have been proven to be suitable candidates. The sensitive material was applied to a capacitor and changes its electrical parameter during the polymerisation. Linseed oil shows significant changes of its dielectric constant of about nearly 40 % depending on the ambient conditions. To reach lower response times, fatty acids with more double bonds are suitable at a comparable behaviour than linseed oil. Ambient conditions such as humidity and temperature influence the reaction speed, but do not significantly influence the threshold statement.
Observed ADC and real capacity values of different chip sizes with
liquid and solid linseed oil (
External energy is only needed during the readout process of the sensor element. This allows for the integration in packages without batteries or energy harvesting. A wireless readout is usually required as the sensor has to be inside the supervised package. Therefore, an RFID interface was developed that allows for the wireless read-out of the capacitance.
The irreversible chemical reaction guarantees high protection against manipulation. Furthermore, the design is simple and thus it allows for low production costs for mass market applications. A miniaturisation and integration are aspired to allow for the usage in smart transponder housings comparable to D14 from microsensys GmbH. The dimension of such a system are only 14 mm in diameter and 2 mm in height. Also a fabrication on foils can be realised, which would allow for the direct integration in packages.
This work was supported by the Federal Ministry of Education and Research (BMBF) by the project “O2-Sens – Niedrigenergie-Sensor zum Nachweis von Sauerstoff in Verpackungen mittels RFID” (contact no. 16SV5277).
The authors thank Jutta Uziel, Birgitt Hartmann, Joachim Döll and Ilona Marquardt from the Technische Universität Ilmenau for supporting the fabrication process and Yahia Cheriguen for his support by creating the stray field simulation models. Furthermore, we thank Ingo Hörselmann from the Technische Universität Ilmenau for his support in electrical measurements. Edited by: J. Zosel Reviewed by: two anonymous referees