We have developed, modeled, fabricated, and tested a passive wireless sensor system that exhibits a linear frequency-displacement relationship. The displacement sensor is comprised of two antialigned Archimedean coils separated by an insulating dielectric layer. There are no electrical connections between the two coils and there are no onboard electronics. The two coils are inductively and capacitively coupled due to their close proximity. The sensor system is interrogated wirelessly by monitoring the return loss parameter from a vector network analyzer. The resonant frequency of the sensor is dependent on the displacement between the two coils. Due to changes in the inductive and capacitive coupling between the coils at different distances, the resonant frequency is modulated by coil separation. In a specified range, the frequency shift can be linearized with respect to coil separation. Batch fabrication techniques were used to fabricate copper coils for experimental testing with air as the dielectric. Through testing, we validated the performance of sensors as predicted within acceptable errors. Because of its simplicity, this displacement sensor has potential applications for in vivo sensing.
Abstract:Total knee arthroplasty is a common orthopaedic procedure conducted in the United States with approximately 700,000 surgeries performed annually. A common complication following total knee arthroplasty is anterior knee pain which affects tens to hundreds of thousands of people each year. The exact mechanism that leads to anterior knee pain remains unknown, but improper component selection may cause pathologic loading of the knee which leads to pain. Measuring loads in the knee to elucidate the mechanisms underlying anterior knee pain remains a challenge because the joints are so small. Using novel wireless sensor technology, we have developed and validated the first "smart" patellar implant capable of measuring force magnitude and force distribution in the knee. Implantable force sensors were calibrated and tested through the range of physiologic loading. Three sensors were then interfaced with a Zimmer patellar implant and placed into a custom loading apparatus. The smart patellar implant was then incrementally loaded from 0-500 N. Sensor signals were all recorded simultaneously in real time to measure the load across the patellofemoral joint. Results demonstrated that the smart patellar implant was able to accurately measure the load being transmitted across the simulated patellofemoral joint.
Acute compartment sy ndrome (ACS) is a true orthopaedic e mergency. The potential seq uelae of an undiagno ed ACS include muscle necrosi , contracture, and in some cases amputation. We have developed a simple, wireles passively-powel'ed sensor that has t he potentjal to provide continuous monitoring of intracompartmental pressllre ill lIillo. This will allow clinicians to ma ke an early d.i agno is of ACS which is essential to preventing equelae and unnecessary fasciotomies. I. I TRODUCTIOAcute compartment syndrome (ACS) is a serious medical condition that ocems when edema causes an increa e in hydrostatic pressure witllin a myo-fa ia l compal1ment. Injuries as ociated with ACS include fracture oft tissue trauma, and crushing injuries. A significant increase tn hydrostatic pressure within the compartment will cause i chemia. If left untreated thi can lead to muscle necro is contracture, and amputation in advanced ca es. Decompressive fasciotomy is an effective technique for preventing ACS . However, thi procedure can result in significant morbidity and complications. Current diagnoses of ACS are made based on c1inka] examination, which is difficult if the patient is obtunded and unable to communicate, or ba ed on measurement of intracompartmental pre ures using needle-manometer systems. Needle-manometer systems are highly technique-dependent making them liable to confound a diagnosi [I]. eedle-manometer ystems resu lts in false negative rate up to 35% [2]. For these rea on , the aim of thi s re earch wa to develop a wireless robust, pas ively powered imp lantable pres ure sensor that can provide continuous monitoring of intracornparLmenlaL pressure. This will provitle a tool [or clinicians to make early diagnose of ACS, tbus preventing potential sequelae and ullnece ary fa ciotomies . 978-1-4799-8360-5/15/$31.00©2015 IEEE n. M T HODS A. Sensor DescriptionWe have developed a pressure ensor tbat is simple, inexpen ive to fabricate wirele and pa ively-powered. The prototype sensor are comprised of a pair of paral lel Archlmedean coils made of conductive metals with an intervening dielectric layer. The spira l pair function as an inductor-capacitor (LC) resonator. When expo ed to a radiofi'equency (RF) field , the seosor resonates at a characteristic frequency. A change in hydrostatic pressure changes the distance between the Archi.medean coil s and thus change the resonant frequency of the sensor. In our prototypes, the intelvenillg djelectric layer is composed of 100% 527 Sylgard (Dow Corning Midland, MT) . Thi was cho en because it defonn ufficiently under phy iologically relevant pressure ranges for ACS. causing measurable shifts in the resonant frequency of the sensor. B. Sensor TestingTo evaluate the sensitivity of our en or technology to change in physiologically relevant pressure range for A S , prototype disk sensors measuring mrn il] diameter were placed in an air-tight, air-filled plunger system. Hydrostatic pressme was then increased incrementally from 0 to 116 mmHg three con ecutive times. The plunger y ...
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