Novel Tattoo Sensors
UHF RFID works like RADAR. The tag is energised by the reader signal and reflects modulated information back to the reader. The system is referred to a passive as no battery is needed on the tag. In favourable conditions (no obstructions) read ranges of 7m+ can be achieved.
The initial hand painted prototype tag was printed direct onto temporary tattoo transfer paper using silver nano-particle ink. It was necessary to perfect the printing processes to produce the high quality tag shown transferred to an arm. The resulting transfer is separated from the skin by a polymer layer approximately 18µm thick.
Dr John Batchelor
John's RFID transfer tattoo design was highly commended in the IET 2011 innovation awards and his work on body-worn antennas has been very well received.
The passive skin-mounted wireless sensing technology used in AART-BC is based on UHF RFID (Radio Frequency Identification) which operates at 860-920MHz. RFID is widely used in systems such as contactless payment and electronic door keys. However, these systems operate at 13.5MHz and are very short range. UHF RFID operates like radar and can give read ranges of a metre or more when mounted directly on-skin. One of its benefits is that no battery is required on the skin-mounted tag.
Normally an antenna would not work well if it is mounted directly on the skin. The tattoos are able to function because they include a slot as part of their design. Much of the energy on the patch is tightly bound around the slot which means it does not impinge greatly on the skin beneath. Therefore, much of the power in the tag antenna is radiated back to the reader.
The slot provides an impedance transformation between the antenna terminals and the complex impedance of the transponder chip. The tag becomes mis-matched when the slot conditions change (the energy entering the tag is reduced). This means more power is required at the reader to activate the tag and this can be used to make the tag into a sensor.
Passive Assistive Technology with Dignity
- Ultra-low profile skin mounted tags with adjustable shapes opens up potential for HCI applications.
- Tags must sense strain to act as muscle twitch sensors.
- Inkjet printing using conducting ink could make this a cheap and convenient process providing conducting element can stretch.
The tattoo tags can be used to produce wheelchair control assistive technology that is not obtrusive or functional looking. This work will be widely applied to assistive technologies in this EPSRC funded project.