The swelling ratio of a network polydimethylsiloxane correlates with the Hansen solubility parameters, δd, δp and δh and the vapour pressures of organic vapours. The use of PDMS as a mechanical actuator in a prototype wireless RFID passive sensor is demonstrated.

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The present work describes the inkjet printing and low temperature sintering of silver nanoparticle inks onto transfer tattoo paper. Our approach results in silver features of excellent resolution and conductivity and, subsequently the first passive UHF RFID transfer tattoo tags functional mounted on human skin of improved performance when compared to screen printed passive UHF RFID transfer tattoo paper tags.Moreover, inkjet printed passive UHF RFID transfer tattoo tags show similar performance to copper etched passive UHF RFID tags on plastic substrates. This study compares the image quality (resolution) and electrical performance of two commercial silver nanoparticle inks inkjet printed on transfer tattoo paper.The optimal printing and sintering parameters to obtain high resolution features of resistivities 20 to 57 times the resistivity of bulk silver (1.59 Â 10 À6 ohm cm) are described. We demonstrate how, by selectively depositing ink in specific areas of the antenna, read distance of passive UHF RFID tags can be increased from 54 to 68 cm whilst decreasing the amount of ink used by 33%. Furthermore, this approach results in inkjet printed passive UHF RFID tattoo tags with larger read distance than silver screen printed passive UHF RFID tattoo tags, 45 cm, and similar to copper etched passive UHF RFID plastic tags, 75 cm. Moreover, inkjet printed passive UHF RFID tattoo tags in this work are considerably thinner (1-5 mm) than screen and etched passive UHF RFID tags (tens of micrometers) hence, making the former more appealing to the end user. In addition to this, inkjet printing is compatible with large area manufacturing techniques and has the potential to evolve as one of the most promising RFID mass-production techniques. Therefore, this work represents a step towards the commercialization of on-body transfer tattoo paper passive UHF RFID tags.

Additive fabrication of electromagnetic structures by inkjet printing technology is both cost effective and compatible with a wide range of environmentally friendly substrates, enabling the fabrication of frequency-selective surface (FSS) arrays with line dimensions less than 0.1 mm, which is difficult to achieve with conventional subtractive techniques. Several approaches, such as savings in ink by depositing it at the edges of dipole elements where the surface current tends to maximize, have been investigated in order to produce low-cost frequencyselective panels with acceptable level of isolation. The FSS transmission characteristics were improved by jetting multiple ink layers on the whole elements and at the edges. The electrical resistance of various arrays have been measured and analyzed and has been used to assess the performances of the FSS.

This letter describes the performance of FSS arrays in which some of the conducting elements at randomly chosen locations were absent or defective. The aim was to assess the proportion of defects that can be tolerated when low-cost fabrication techniques are employed.

A route to cheap and disposable sensors for the chemical sensing market, with potential applications including monitoring of food spoilage, is reported herein. The sensor is the result of the direct integration of a stimuli-responsive material, poly(dimethylsiloxane) (PDMS), with an electronic component. The printing and sintering of colloidal silver ink solutions onto PDMS was optimized to allow the printing of conductive silver feed loops, which are the active sensing component in antennas for passive (battery-free) Radio Frequency Identification (RFID) tags. The response of these devices is related to the degree of swelling of the PDMS, which, in turn, has been shown to be correlated to the Hansen solubility parameters and the vapour pressures of the corresponding volatile organic compounds (VOCs). When exposed to solvent vapour the printed feed loop fractures, increasing resistance and ultimately breaking conductivity, leading to a change in the transmitted power and read range of the wireless device. Remarkably upon removal from the vapour, the fractured feed loops reassemble and become conductive again, making them switchable and ''multi-use''. This work paves the way to a fully inkjet printed RFID substrate for vapour detection.

A passive smart tag is described that responds to dampness in diapers once a pre-defined threshold value is reached. A high-frequency (HF) system at 13.56 MHz is used as this allows operation through water or human tissues with less absorption that would occur for an ultra-HF signal. A circular spiral coil and swelling substrate facilitate a reaction to dampness that can be detected without contact to the diaper wearer. A prototype design is simulated and measured results are provided together with a demonstration of a tag integrated into a worn diaper.1. Introduction: Average life expectancy worldwide in 2025 will reach 73 years [1], with the elderly population expected to rise faster than the general population [2]. Longevity related disabilities and diseases will put more pressure on healthcare. In this Letter, our aim is to design a cheap flexible sensor embedded in a diaper to reduce the workload of carers in nursing homes by logging and notifying staff of the urinating habits of elderly residents.Wireless technologies are already contributing in many biomedical applications and there have been a number of recent studies reporting wireless diaper dampness detectors. In [3], a GSM network communication is used to a tele-monitor in the diaper comprising a processor and a humidity sensor, whereas [4] utilises Zigbee as the transmission system to a sensor including a signal converter, Zigbee module and power supply. A radio-frequency identification (RFID) system is proposed at ultra-high frequency (UHF) in [5] to help juveniles overcome nocturnal enuresis and for geriatrics with incontinence problems. A commercial UHF tag was placed in a diaper and was observed to decrease in function when it became surrounded by liquid and saturated superabsorbent gel. RFID systems are similar to barcoding, and can identify a tagged object. They however can carry more information and therefore are used in many logistics and asset tagging applications where wireless and non-line of sight identification is required [6]. RFID tags are also detectable when obscured by dirt, covered by polymer housing or even embedded with an object to be identified [7][8][9]. Moreover, external components and microelectromechanical system sensors can be added for additional functionality [10]. However, the RFID system performance can be influenced by materials such as metals and water in the environment in which the tag has been applied because the wireless transmission is degraded with a subsequent reduction in the tag read range [7,9,11].Rather than using the UHF band discussed in [5], we propose a tag using high-frequency (HF) RFID, which transmits by induction between two coils and is widely used, often at 13.56 MHz, in short range applications such as NFC [12] and the MiFare system [13]. The use of magnetic coupling to communicate is advantageous in the presence of the human body which is highly capacitive and causes significant tuning problems in conventional antenna systems such as those at UHF. Experimental studies of HF tag performance h...

This paper addresses the antennas design for passive UHF rfid applications involving the human body as the object to be tagged. Novel antenna geometry is proposed for conjugate impedance matching to the complex impedance of the rfid transponder. The proposed tag is capable of achieving high read range directly attached to a human thorax or in free space.