Research Papers

Battery-Less Wireless Instrumented Knee Implant

[+] Author and Article Information
R. Rajamani

Department of Mechanical Engineering,
University of Minnesota,
Minneapolis, MN 55455

J. E. Bechtold

Department of Orthopaedic Surgery,
University of Minnesota,
Minneapolis, MN 55455

Manuscript received April 28, 2012; final manuscript received January 2, 2013; published online February 11, 2013. Assoc. Editor: Vijay Goel.

J. Med. Devices 7(1), 011006 (Feb 11, 2013) (11 pages) Paper No: MED-12-1057; doi: 10.1115/1.4023412 History: Received April 28, 2012; Revised January 02, 2013

Over 400,000 total knee replacement procedures (TKR) are performed annually in the United States. This paper focuses on the development of a battery-less wireless instrumented tibial tray for performance feedback in TKR implants. The proposed instrumented tibial tray is powered internally by an integrated piezoelectric energy harvesting system. Energy is harvested during the walking of the patient when forces are exerted on the tibial component. The sensors and wireless electronics are entirely powered from the harvested energy. This tibial tray is also instrumented with capacitive force sensors and an ultra low-power method to measure the capacitive force sensors. A bench top test rig is developed for testing the battery-less wireless knee replacement implant. For a person with a body weight of 55 kg, the energy harvesting system can fully charge the storage capacitors in 11 steps and can harvest an average of 1051 μJ per step. To power the force measurement system for ten seconds and to transmit the data, the piezoelectric energy harvesting system must be charged before the force measurement process is initiated by a minimum of 11 steps and a minimum of two steps must be taken during the force measurement process. During the force measurement process, each force sensor is sampled at a frequency of 10 Hz for ten seconds; thereafter, all of the data is transmitted to the RF base station. The resulting capacitive force sensors adequately represented cyclic loads; however, the sensors demonstrated some issues with repeatability.

Copyright © 2013 by ASME
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Fig. 2

Total knee replacement implant

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Fig. 3

Layout of instrumented tibial tray

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Fig. 4

Schematic of typical PEHS circuit

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Fig. 5

State diagram of energy harvesting system

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Fig. 6

Schematic for RC time constant measurement circuit

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Fig. 7

Diagram of concept for a capacitive sensor

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Fig. 8

Graph showing nonlinear relation between dielectric thickness and sensor sensitivity

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Fig. 9

Final design of capacitive force sensor

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Fig. 10

Arrangement of force sensors in tibial component

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Fig. 11

Cross-section of prototype knee implant showing layout of various components

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Fig. 12

Pictures of tibial component showing circuit board, ground plate, and compliant material for sensors

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Fig. 13

Picture of knee load application device

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Fig. 14

Typical loading cycle with knee load application device at 276 kPa air pressure

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Fig. 15

Typical charging cycle for 836 N nominal peak force

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Fig. 16

Relationship between harvested energy of PEHS and applied force

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Fig. 17

The top graph shows the force linearly increasing on the piezo transducer. The middle graph shows the voltage on the input capacitor which is coupled to the piezo transducer by a full-wave rectifier. The bottom graph shows the voltage on the storage capacitor.

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Fig. 18

State diagram for capacitive sensor measurements

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Fig. 19

Relationship between discharge time and capacitance

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Fig. 20

Charge/discharge cycle for capacitance measurement

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Fig. 21

Relationship between measured sensor capacitance and force

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Fig. 22

Force measurements for sensor #1, after calibration completed

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Fig. 23

Storage capacitor voltage while measuring sensors at 10 Hz

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Fig. 24

Storage capacitor voltage while transmitting data




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