The reliability and performance of passive UHF RFID tags must be fully explored before such technology can be depended upon in environments such as hospitals where gamma radiation is used for sterilization.
Radio-frequency-identification (RFID) systems have seen widespread use in supply-chain management as well as in healthcare industries.1-3 Hospitals have been investing massively in information technology (IT) to reduce operating costs and improve patient safety, and RFID is expected to become critical to healthcare organizations in achieving these goals. But passive UHF tags used to track equipment and inventory may need to withstand gamma radiation at levels of 25 to 40 kGy typically used for sterilization.4,5 This report will evaluate the reliability of passive UHF RFID tags in such a high-radiation environment.
For hospital equipment and materials subject to gamma radiation for the purpose of sterilization, the RFID tag and its internal integratedcircuit (IC) chip and antenna must withstand that radiation in order to effectively track that equipment and material. RFID tags currently on the market are not well suited for such a high-radiation environment; for a tag to withstand gamma radiation, it must have the capability to be hardwired. Testing RFID technology in a medical environment can be difficult because of the complexity of the environment, as cases in implementing information technology (IT) in hospitals have shown.6,7
In a hospital, the adoption of RFID may not necessarily be as involved as in supply-chain applications, since medical services rely more on staff and internal processes than on external suppliers. Nevertheless, any organization that plans to adopt RFID must face multiple challenges, which may include the following technological, managerial, and organizational problems. Sarma9 considers three major challenges, mainly from the technical viewpoint: non-line-of-sight reading, handling of serial numbers, and handling large volumes of realtime data. Sarma notes that solutions may depend on building an RFID infrastructure, together with middleware and impedance-matching of the RFID system to current systems such as Enterprise Resource Planning (ERP) systems.
The medical and life-science fields have deployed gamma radiation for a variety of applications, including sterilization and removing potential infestation of insects and bacterial contamination in produce imported from other countries. Early experimenters working with item-level RFID tags have discovered that gamma radiation levels used in typical sterilization cycles can permanently damage or affect the data and electronic memory contained in an RFID tag. A gamma radiation level of 25 kGy is typically used for sterilization of disposable medical items, while some medical institutions will use levels as high as 40 kGy for their sterilization procedures. Gamma radiation consists of the emission of massless particles called photons as a result of the decay of a radioactive material. Gamma radiation has a relatively long wavelength and can be stopped by a lead barrier (Fig. 1), in contrast to heavier forms of radiation, such as alpha and beta rays, which are more dangerous when in contact with living organisms. Gamma radiation falls within the same electromagnetic (EM) spectrum as visible light, ultraviolet light, and infrared light, but with much higher energy levels. Medical devices are often sterilized by using gamma radiation emitted by cobalt- 60 as a radiation source.
As defined by the United States Food, Drug, and Cosmetics Act, a medical device is "an instrument, apparatus, implement, machine, contrivance, implant, in vitro reagent, or other similar or related article, including any component, part or accessory, which is intended for the use in the diagnosis of disease or other conditions, or in the cure, mitigation, treatment or prevention of disease, in man or other animals, or intended to affect the structure or any function of the body and which does action and which is not dependent upon being metabolized for the achievement of its primary intended use."9
RFID is an Auto Identification and Data Capture (AIDC) technology that makes use of omnidirectional wireless radio communications to transfer and store identification data, unlike the line-of-sight operation of barcode scanners.10 Passive RFID transponders or tags are small, resourcelimited devices that are inductively powered by the energy of the request signal sent by an RFID reader; this is known as the forward communication or downlink signal. Once the RFID tag receives enough energy to "power up" its internal electronics, the tag can decode the incoming query and produce an appropriate response by modulating the request signal using one or more subcarrier frequencies. The tag responds by employing the use of backscatter or uplink communication. These RFID tags can do a limited amount of processing, and have a small amount (<1024 b) of storage. Semi-passive and active RFID tags require a battery for operation, but provide more functionality. Battery-powered RFID chips present fewer security and privacy challenges than passive ones. (See the follow-up article for more details on the types of technologies employed in RFID systems.)
Three general categories are being practiced in healthcare environments first established in retail inventory. The first is as a control setting for improving asset management of drugs and medical devices, to ensure that a patient receives the correct drug in a timely fashion. The second is for the sterilization of hospital equipment using gamma radiation. The third is automatic capture of streaming data to ensure that at any time, reliable data is available for clarification of medical questions. RFID tags and RFID badges allow hospital managers to locate and effectively deploy staff and assets such as crash carts, portable defibrillators, wheelchairs, and infusion pumps. Previous deployments in this area required expensive active tags containing a battery and high infrastructure costs. Recent market developments are allowing passive tags to provide much of this functionality, and overcome any concerns around patient and staff privacy. Apart from sterilization applications, RFID has the capacity of reducing the labor cost of scanning items; reduction of out-of-stock items; reducing theft loss; and providing proof of delivery, inventory reduction, and facilitating promotions at stores. Recently, researchers at the Cantonal Hospital of St. Gallen (St. Gallen, Switzerland) have found that high-frequency (HF) 13.56-MHz RFID tags do not significantly interfere with the functionality of imaging devices, nor do those imaging devices affect the functionality of the HF RFID tags. The study, undertaken by physicians and researchers at the hospital and at the University of ETH Zurich, found that magnetic resonance imaging (MRI) radiation could raise the temperature of tissue around an RFID tag by, at most, 4C (7F), but had no effect on a patient's health. RFID has been observed to interfere only in analog hospital equipment, although most newer communications devices are based on complex digital modulation formats that tend to be more robust in the presence of interference.
RFID technology consists of tags, readers, computer networks, and other systems that may include middleware, databases, and software. The RFID industry consists of suppliers of these various components as well as systems integrators. With their long wavelengths, the range of UHF RFID systems is often detuned in the presence of water and metal objects. Healthcare environments often experience high humidity, for example, because of water baths. Any RFID solution used for healthcare applications, for tagging, sterilization, or even inventory management, must be rugged enough to withstand such a harsh environment.
RFID systems, interrogators, and tag performance test methods are covered under the three parts in ISO/IEC 18046. The most common methodology used in the testing of RFID tags is in a free-space environment. This test approach measures the response rate and attenuation of tags by varying the distance (d) between the tag and the reader. In experiments reported here, collected by the a tag's antenna in free space (Pa) can be stated in Eq. 1 as
Pa = tGt/4πr2)(λ2/4π)> (1)
Pa = the total power received by an RFID tag's antenna in free space,
Pt = the power of the transmit antenna,
Gt = the gain of the transmit antenna,
λ = the electromagnetic wavelength of the RFID's antenna, and
r = the distance from the transmitter.
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Equation 1 shows that the power received by an RFID's antenna is affected by antenna gain, signal wavelength, and distance from the transmission source. 13
The power available at an antenna, Pa, is a function of various factors including power and gain (efficiency) of the transmitter antenna (Pt and Gt), the distance from the transmitter (r), electromagnetic wavelength (?) and gain (efficiency) of the RFID tag's antenna (Gtag). To improve the antenna read range (r) without increasing transmitted power, it is necessary to increase the gain of the antenna.
Often the characterization of RFID tags involves characterization of the antenna over a wide range of frequencies. 14-16 The RFID reader uses the re-radiated power to demodulate the signal from the reader. The greater the re-radiated power from the tag, the better for the reader to decode the signal from the RFID tag. The re-radiated power is also influenced by factors such as antenna gain and tag-antenna impedance matching. Equation 2 expresses the reradiated power as a function of several factors, including tag gain, Gtag :
Re-radiated power = (4Ra2)/(Za + Zc2) PaGtag (2)
Za = the RFID antenna impedance and
Zc = the RFID chip impedance.
Equation 3 shows that reradiated power is highly dependent upon the impedance matching between the inlay and the RFID tag's antenna:
K = 4Ra2/|Za + Zc|2) (3)
When the impedance of the antenna is zero (a short circuit), the tag reradiates four times as much power as a matched antenna. When the antenna impedance is highly reactive (a capacitance), a complex-conjugate loaded antenna actually reradiates more power than an antenna with zero impedance.14 The antenna and inlay impedance/reactance can have an impact on RFID tag performance. Other factors that can improve the efficiency of an RFID are described in Eq. 4 ,which shows that the maximum read range (Rangemax) is a function of distance and equivalent isotropic radiated power17
Rangemax = d(EIRP/Ptag-minGtag)0.5 (4)
Equation 4 suggests that the read range of an RFID interrogator or reader is a function of the distance from the tag and the EIRP.14
Additional factors that can affect the read range of an RFID reader are summarized in Eq. 5:
Rangemax = (λ/4π)tag-min Greceive-tag-minτ)0.5/Ptag-min)> (5)
In Eq. 5, t is the same K factor as in Eq. 3. The read range of an RFID reader can be theoretically estimated from wavelengths, power, and gain coefficients but, in reality, it is difficult to achieve because of many environmental factors. The tag and RFID chip impedance are only two of many factors that affect the read range.13,14,17 The materials from which the tags are made and the form factor can also affect the read range of an RFID antenna. A practical example is a reader with a range of 3 m in one environment and less in another environment.18-20 The reason for this may be linked to the impedance of the tag's antenna and RFID chip (as noted in the parameter t of Eq. 5). An RFID antenna's impedance can be affected by the tag's substrate even when materials with similar EIRP are used to build the tag, since changes in impedance can affect the read range.
Most of the RFID tags currently on the market cannot withstand high radiation levels. (For a review of typical passive RFID tag construction and components, see the accompanying article.) For an RFID tag to withstand high levels of radiation, it must be capable of being hard wired. A gamma-radiationresistant tag must be made with materials capable of withstanding the harsh radiation environments typical of the sterilization units of most medical applications as well as the radiation levels that are commonly found in and around the radioactive waste produced in nuclear power plants.
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Gamma-radiation-resistant RFID tags have the capability of tracking healthcare equipment that has been sterilized by means of gamma radiation. Also the IAEA is willing to certify some countries to electricity from radioactive materials. Minimum contacts can be guaranteed between the materials and human beings by deploying gamma tags in the industry.
The authors would like to thank SENSTECH (PKT 1/2010) for supporting this work.
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