Battery-Free Smart Sock for Abnormal Relative Plantar Pressure Monitoring
Abstract:
This paper presents a new design of a wearable plantar pressure monitoring system in the form of a smart sock for sensing abnormal relative pressure changes. One advantage of this approach is that with a battery-free design, this system can be powered solely by radio frequency (RF) energy harvested from a radio frequency identification (RFID) reader unit hosted on a smartphone of the wearer. At the same time, this RFID reader can read foot pressure values from an embedded sensor-tag in the sock. A pressure sensing matrix made of conductive fabric and flexible piezo-resistive material is integrated into the sock during the knitting process. Sensed foot pressures are digitized and stored in the memory of a sensor-tag, thus allowing relative foot pressure values to be tracked. The control unit of the smart sock is assembled on a flexible printed circuit board (FPC) that can be strapped to the lower limb and detached easily when it is not in use. Experiments show that the system can operate reliably in both tasks of RF energy harvesting and pressure measurement.
Existing System:
In-shoe pressure systems have pressure sensors in shoes [11] or embed sensors into socks/insoles [7], [12], and is thus more appropriate for measuring a dynamic pressure distribution while the wearer is walking or standing [7]. They are more portable as compared to platform systems and have been successfully implemented in previous studies for diagnosing diabetic foot [12] or collecting pressure values [7], [11]. However, for such systems that target wearable applications, the majority of their controller designs are using rigid printed-circuit boards (PCBs) housed in solid enclosures [7], [12], or have their sensors partially protruding above the surface of the insole [11], which can make wearers feel uncomfortable. The accuracy of the in-shoe systems can also be affected by any spaces created between the foot and underlying sensing elements due to some surface mismatch between the foot and the insole or as a result of loose wearing [5], [8].
Proposed System:
The proposed sock system converts the RF energy of the interrogation signals radiated from a smartphone-hosted radio frequency identification (RFID) reader into dc to power the system. At the same time, the RFID reader receives digitized foot pressure data stored in and transmitted from the memory of the RFID tag. The received data can be displayed on the screen of patient’s smartphone for self-monitoring or forwarded to his/her caregiver for remote monitoring purpose. Besides, unlike other wearable monitoring systems that used rigid PCBs [7], [11]–[13], both central control unit and antennas are carried by flexible printed circuits (FPCs) that can be mounted and demounted easily using reliable snap fasteners on a stretchable interface fabric. In the smart sock application, these antennas will be worn close to human body, which is fluid-rich and exhibits a large dielectric-constant at microwave frequencies [18]. Thus, the performance of antenna will be significantly affected [19]. To mitigate the body effect, one option is to increase their separation so that less electromagnetic (EM) energy will be absorbed by the body tissues [20]. Another option is to intentionally detune the antenna in free space such that when it is worn close to the human body, it would be retuned automatically to the desired resonant frequency.
Conclusion:
This paper presents the design of a new sock system, which monitors for abnormal changes in the wearer’s foot pressures. In this design, a textile-integrated pressure sensor matrix is knitted inside the sock itself, and RFID technique is applied to collate the sensed readings. Besides, the system is powered only by harvested RF energy radiated from a smartphone-hosted RFID reader in order to realize a battery-free system. Furthermore, a sharable antenna system, which composes of two orthogonallyoriented chip antennas, is used for both energy harvesting circuit and the RFID sensor tag on the sock. The FPC, which carries the electronic circuitry of the system, can be mounted on and demounted from the sock easily using snap fasteners on an interface fabric connector that is stretchable to fit different users. The measurement resolution of the system is found to exceed the typical requirement for plantar pressure measurement in real applications.
Reference:
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