Radio Frequency Identification as a Testbed for Integration of Low Frequency Radio Frequency Sensors Into Orthopedic Implants

Abstract

Recent advances in radio frequency (RF) sensor systems provide new opportunities to wirelessly collect data from inside the body. “Smart implants” instrumented with sensors have been used as research tools for decades, but only recently have implantable sensors become small enough and robust enough to be used in daily clinical practice. In orthopedic surgery, implants provide a vehicle onto which small RF sensors can be mounted to gather data for diagnostics. However, the sensors must function in a challenging environment which requires long term functionality under demanding physical and mechanical conditions. The purpose of this study was to parametrically test low frequency RF systems under simulated in vivo conditions to determine feasibility of sensor integration into orthopedic applications. Three low frequency RF systems were tested in several clinically relevant scenarios in vitro to characterize (1) strategies for maximizing communication range, (2) physical robustness, and (3) mechanical performance. Systems were tested in air, saline, soft tissue, bone, and in proximity to metal. Hermeticity was assessed during a 208 week period. Effects of γ-irradiation and repeated steam sterilized were measured. Strain at failure was measured by mechanical testing of various packaging configurations. All systems were capable of greater than 20 cm read range under ideal conditions. Saline, soft tissue, and bone had minimal effect on signal transmission, but read range was sensitive to the proximity of stainless steel. The electronics were tolerant of steam sterilization but not of γ-irradiation. Polymer encapsulation is robust enough for many orthopedic applications, but ceramic encapsulated sensors need to be optimized for weight-bearing applications to avoid brittle failure. Although sensor packaging remains a challenge, the technology exists to incorporate passive wireless implantable sensors into orthopedic daily practice.

Smart implants in orthopedic surgery, improving patient outcomes: a review

“…Owing to the relatively large physical size of many orthopedic implants, the bulk provides an opportunity for symbiosis between implant and sensing technology. 6 Physically large implants provide the means to incorporate sensors, signal conditioning electronics and telemetry into the implant itself or on its surface. Because of the opportunity for integration of sensing technology, there has been much innovation and development in smart orthopedic implant applications over the last 5 decades.…”

“…For systems with complex electronics, technical challenges include power consumption, communication range, data transfer rates, size, robustness and cost. 5 , 6 , 96 , 97 To address power consumption challenges, ultra-low power circuits coupled with energy harvesting strategies have been explored. 24 Using these strategies, energy is generated in the implant from sources such as vibrations, rotations and deformations during activities such as walking.…”

“…Perhaps, the most significant barrier to integration into clinical practice has been the necessity for host implant modification to accommodate sensors and electronics. 5 , 6 Creating hollow cavities to house complex electronics and strain gages ( Figure 3 ) is technically challenging and expensive and, most significantly, alters the properties of the implant and ultimately jeopardizes the implant’s performance. 6 Sensors for next-generation smart implants will be small, simple, robust and inexpensive and will necessitate little to no modification to existing implant designs.…”

“…Passive resonators are an alternative to traditional sensor systems because passive resonator sensors do not require power or signal conditioning electronics. 5 , 6 , 100 , 103 , 104 Passive resonator sensors are generally small, simple and comprise few components. They do not require signal conditioning or telemetry because when they are exposed to an RF field (via an antenna), they resonate.…”

Bio-compatible wireless inductive thin-film strain sensor for monitoring the growth and strain response of bone in osseointegrated prostheses

“…Their work proposed the use of wireless inductive coupling to deliver energy to and read data from an implanted sensor using an external reader. More recently, Korduba et al 14 explore the use of miniaturized radio frequency identification (RFID) platforms for in vivo application. The compact devices are small in size (largest dimension is 12 mm) and use commercial RFID solutions embedded into glass or ceramic packages.…”