Embedding of electronics as a part of versatile structures is a growing trend, especially along the emerging paradigm of the Internet of Things (IoT). Many industries are interested in novel smart products with identification and sensing possibilities.
In addition to logistics and supply chain control, great potential lies especially in the construction, packaging, and furniture industries, where wood is a typical material. This makes the integration of electronics and wood, especially plywood, a very interesting research area.
Passive radio-frequency identification (RFID) technology provides automatic identification and tracking of products achieved with battery-free remotely addressable electronic tags composed of an antenna and an integrated circuit (IC) on a substrate.
Electromagnetic waves are used by an RFID reader and an RFID tag to communicate with each other. Furthermore, the electromagnetic waves sent by the reader are used to provide operating power for passive RFID tags as they have no power supply of their own.
The use of propagating electromagnetic waves in the ultra-high frequency (UHF) frequency range for powering and communicating with the passive tags enables rapid interrogation of a large quantity of tags through various media.
Unlike barcodes, the IC-enabled tags allow the data stored in them to be updated or read wirelessly at any time. Thanks to the energy efficient mechanism of digitally modulated scattering utilised in the wireless communication, the tags can be read from a distance of several meters.
This makes the passive tags favourable to be used for energy-autonomous wireless sensing platforms, for example, as strain, temperature, and humidity sensors that exhibit low complexity and cost.
Maintenance-Free, Embeddable Sensor
It is possible to establish maintenance-free embeddable sensor components into different kinds of structures without the need for external sensors or on-board electronics, by using a passive UHF RFID tag antennas as the sensing element.
Antenna-based sensing provides integration of sensing capabilities in passive RFID tags with a minimal increase in the overall complexity and power consumption of the tag.
One of the most interesting sensor types for passive RFID systems is a humidity sensor, which is due to the great number of materials reacting to humidity-level variations. In addition, the information about possible existing humidity or knowing the humidity level is essential in various application areas, including the construction industry, warehousing, and transportation.
Previously, humidity sensor prototypes have been developed for UHF RFID systems by using traditional photolithography, as well as inkjet and screen printing.
The passive humidity sensor tag in this case is fabricated directly on plywood substrate by using brush-painting and photonic sintering of cost-effective silver ink.
The sensor tag is still at a prototyping stage but the tag is intended to be used for structural humidity monitoring, for example, inside wooden floors and walls, or to sense humidity conditions during transportation.
The sensor tag can be used to quickly detect increased humidity level, therefore, dramatically reducing the amount of additional damage caused by continued humidity exposure.
The sensor tag is fully passive, does not need any maintenance procedures, and can be permanently enclosed inside walls, ceilings, floors, and wooden containers for long-term monitoring. In addition, the sensor tag is small in size, allowing fitting inside various structures.
This sensor tag also has all the functionalities of an ordinary passive RFID tag, which allows the use of several sensor tags in a small area, as they can be recognized using their unique identification codes. The tags can also be used for automatic identification and supply chain control of the wooden products.
Characteristics Of Birch Plywood
Plywood is characterised by its high planar shear strength and impact resistance, excellent surface hardness, wear resistance, and creep resistance.
Other good characteristics of birch plywood include being flexible and bendy, being strong and durable, easy working properties, good gluing properties, being ecological and biodegradable, and not to forget the beauty of its surface. All these properties make birch much used and loved material, for example, in building and furniture industries.
Plywood has excellent dimensional stability under heat. In practice, the thermal deformation of plywood is so small that it can generally be disregarded.
Standard Finnish plywood and most coated plywood products are suitable for use at temperatures of 100deg C and many up to 120 degC. Plywood endures cold even better than heat and can be used at sustained temperatures as low as −200 degC.
Although plywood burns, it can have better fire resistance than many materials which do not burn. The temperature at which plywood will ignite when exposed to a naked flame is about 270degC and to cause spontaneous combustion, a temperature of over 400 degC is needed. Therefore, the temperature area of plywood is very suitable for the fabrication methods used.
Like all other wood-based materials, plywood is a hygroscopic product and exhibits viscoelastic mechanical behaviour. An increase in moisture content will result in a decrease in the strength, modulus of elasticity, and shear modulus values. Therefore, also for these reasons, it is necessary to take the moisture conditions into consideration when using plywood.
Brush-painting is a versatile but simple and fast additive manufacturing method. The method not only reduces the process-steps of RFID tag manufacturing, but also minimizes the need of conductive ink material, as the material is dispensed directly to the brush and from the brush directly to the antenna area in the substrate.
By brush-painting RFID tags directly on plywood, we can manufacture very thin tags through eliminating the need for additional substrate material. The thickest part of the tag in this case is the IC, which determines the scale of the thickness of the tag.
In this case, the thickness of the IC is 120 μm, but the use of even thinner ICs, for example, 75 μm, is possible, meaning that embedding the tags inside versatile products is convenient. In addition, when the tags are embedded as a part of the wooden product, they will be almost impossible to remove from the product without breaking it, as the wooden item itself acts as the substrate of the tag.
Brush-painting has previouslybeen successfully used for fabrication of tag antennas on fabric substrate with silver nanoparticle ink, and on wood substrate with silver and copper nanoparticle inks.
In this case, cost-effective screen printable silver ink was used. The tag antennas were brush-painted through a stencil (50 μm thick polyimide film) on plywood substrate, by using only one layer of ink. A tag antenna was utilized as the antenna geometry. The wood substrate, in this case, was 4 mm thick birch plywood.
Efficient and low-cost manufacturing is more and more important, as a huge amount of RFID components is needed for future IoT applications. These requirements mean that the long sintering times needed in heat sintering are not acceptable.
Flash lamp sintering is a photonic sintering method that in ambient conditions uses very short light pulses to heat the ink to a high enough temperature within few micro- or milliseconds. Such transient heating minimizes the damage to heat sensitive substrates.
Photonic sintering reduces the sintering times significantly, compared to the widely used heat sintering, which can take tens of minutes. The photonic sintering was done using Xenon Sinteron 2010-L system. The sintering system parameters are lamp voltage, flash pulse duration, and number of flash pulses.
The lamp voltage can be adjusted between 1800 V and 3000 V, in 50 V increments. The pulse duration can be adjusted and the time between pulses can also be chosen.
In order to find the optimised sintering parameters, the resistances of fabricated antennas were measured after sintering by placing the measurement probes on the corners of the antenna pattern.
In order to achieve reliable and durable sensor tags against various environmental conditions, but still enabling their efficient use as sensors, the shielding of the IC and the antenna needs to be considered. Therefore, finally, regular water-proof glue was brush-painted as a protective coating over tag antennas and ICs.
According to the moisture absorptiontest results, the mean change in mass was between 15 percent (all-coated) and 23 percent (IC-coated) already after five minutes in 100 percentrelative humidity (RH), which means that the plywood material quickly absorbs a lot of moisture.
After one hour in 100 percent RH, the mean change in mass was between 31% (all-coated) and 46% (non-coated), which means that about half of the mass increase occurs during the first five minutes, and the degree of moisture saturation is most probably near 100 percent after one hour. However, even though the plywood absorbs quickly a lot of moisture, it also dries after removal from moist conditions.
In the mass measurements after nine days, it can be seen that only a very small amount of moisture is still absorbed. These changes in mass after nine days, compared to the initial measurements, are so small that they could also be due to changes in the moisture content of office conditions. The next step is to conduct exact moisture level measurements that will be used in addition to these prototype stage moisture absorption measurements.
The read range results for non-coated, IC-coated, and all-coated tags (after nine days in office conditions, after adding the possible protective coating, immediately after 100 percent RH testing, and after nine days in office conditions) showed that these tags initially achieved peak read ranges of 10-11 metres in 940–970 MHz, which is more than sufficient for embedded humidity sensor applications.
The achieved results are in line with the earlier results realised with the same antenna geometry: brush-painted silver nanoparticle RFID tags with the same antenna geometry on polyimide substrate showed peak read ranges of over nine metres in the frequency range of 940–970 MHz (with photonic sintering) and over eight metres in the frequency range of 940–970 MHz (with heat sintering).
Also, read ranges of about five and three metres throughout the global UHF RFID band were measured for brush-painted silver and copper nanoparticle RFID tags, respectively, with the same antenna geometry on wood substrate. For these tags, there was no clear peak read range at any frequency.
Challenging Work Surface
A wooden surface was found to be a challenging surface for brush-painted nanoparticle, inkjet-printable inks, due to its porosity and high surface roughness, as the ink droplets were easily absorbed by the substrate. This problem can be avoided by using screen printing inks.
However, if antenna geometries with really narrow and precise conductors are needed, use of inkjet-printable nanoparticle inks as sensor antenna could be studied with an additional substrate material on top of the plywood.
The measurement results showed that after being exposed to 100 percent RH, the performance of the tags changed significantly. The most dramatic change can be seen for the non-coated tags. The maximum read range decreased from about 10 metres to below four metre, and no peak in read range in any frequency could be seen.
The read ranges of the non-coated tags changed immediately after five minutes exposure to 100 percent RH, but the change was not as radical as after one-hour exposure time. After nine days in office conditions, the performance of the tags returned back to normal, as the tags had dried. These results of wireless read range measurement are supported by the earlier mass change measurement results.
The 100 percent RH exposure also affected the read ranges of the IC-coated tags after five minutes of humidity exposure, and even more after one hour in 100 percent RH. The performance of the IC-coated tags is very similar to the non-coated tags. After one hour in high humidity, the performance of the IC-coated tags seems to be better than the non-coated tags, which is probably due to the effects of moisture on the non-coated IC in case of the non-coated tags.
The all-coated tags endured 100 percent RH exposure quite well: after one hour in 100 percent RH, the peak read range was three metres shorter than before any humidity testing. However, the peak frequency changed to significantly lower frequency.
Altogether, the all-coating of tags shields the tags very well against 100 percent RH, and the reliability of the tags in normal RFID identification applications can be significantly increased by coating. After nine days in office conditions, also the performance of the IC-coated and all-coated tags returned back to normal, whose results are also supported by the mass change measurement results.
In measurements of the IC-coated and all-coated tags, it could be seen that the coating of the IC-area affects the read range up to some extent. The peak read ranges of these tags were slightly shorter after coating than before coating. This change is in the range of about one metre and therefore, acceptable. The read ranges are more than sufficient to the intended applications even after the coating process.
According to the results, the moisture content of the substrate affected the passive UHF RFID tag performance on plywood substrate. The moisture did not prevent the tags from working, although the tag antenna impedance and the ohmic losses were affected by the moisture.
These results are very encouraging to further investigate the moisture sensing based on brush-painted passive UHF RFID tags on plywood substrate. The relation of the humidity exposure time and the change in the read range, especially in case of the non-coated tags, indicated that the tags could be used as humidity sensors in addition to normal RFID identification purposes.
If the tags are intended to be used only as normal RFID identification tags in a high humidity environment, the all-coated tag should be chosen, because of the remarkable reliability increase against humidity.
Based on the results, adding the protective coating on only the IC-area does not give any significant benefit either in use as a humidity sensor or in normal use as an identification tag: the reliability of the all-coated tags is significantly better and the sensor sensitivity to humidity is greater without any coating. However, a longer-term exposure to high humidity must first be studied.
Next, the tags were measured behind a four cm thick wooden wall and under a four cm thick wooden layer. Based on the measurements, the wooden material on top of the tag or in front of the tag had a decreasing effect on the read range, and the peak frequency changed to lower frequency. In addition, the peak frequency is not as clear as without a wood layer.
However, all tags still achieved read ranges of 6–9 metres throughout the global UHF RFID band. According to these results, the fabricated tags are suitable to be embedded into various wooden products and into wooden structures. It should be noted, however, that all of the measurements were performed in situations where there were no adjacent tags in close proximity.
In practical applications, several tags may lay in close proximity. As tags are brought closer to one another, their operation characteristics can alter significantly. If the mutual coupling effect is not taken into account when placing the embedded tags, the readability of the tags could be degraded.
The results showed that the fabricated RFID-based humidity sensor components have a great potential to be utilised in humidity sensing applications but also in automatic identification and supply chain control of various wooden products, especially in the packaging and construction industry.
In the next stage, the sensors will be optimised for the readout with an off-the-shelf RFID reader operating in a fixed regionally regulated frequency band. Also the use of more cost-effective copper-based inks will be further studied.