Research Papers

ROBOImplant II: Development of a Noninvasive Controller/Actuator for Wireless Correction of Orthopedic Structural Deformities

[+] Author and Article Information
Jonathan A. Liu, Mozziyar Etemadi

UCB/UCSF Joint Graduate Group in Bioengineering

James A. Heller

Department of Bioengineering, and Therapeutic Sciences,  University of California, San Francisco, CA 94158

Dillon Kwiat

Department of Pediatric Surgery,  University of California, San Francisco, CA 94143

Richard Fechter

Department of Surgery,  University of California, San Francisco, CA 94143

Michael R. Harrison

 Emeritus of Surgery, Pediatrics Obstetrics, Gynecology and Reproductive Sciences, Director Emeritus, Fetal Treatment Center, Division of Pediatric Surgery, Department of Surgery, University of California, San Francisco, CA 94143

Shuvo Roy1

Associate Professor of Bioengineering and Therapeutic Sciences, Biomedical Medical Devices Laboratory, Department of Bioengineering and Therapeutic Sciences,  University of California, San Francisco, CA 94158shuvo.roy@ucsf.edu


Corresponding author.

J. Med. Devices 6(3), 031006 (Aug 14, 2012) (5 pages) doi:10.1115/1.4007183 History: Received January 31, 2012; Revised July 14, 2012; Published August 14, 2012; Online August 14, 2012

An implantable actuator and its accompanying driver circuit are presented for the purpose of lengthening bones and correcting skeletal deformities without requiring physical contact between the operator and the implanted device. This system utilizes magnetic coupling to form a magnetic gear, allowing an external motor to drive an implantable telescoping rod. The accompanying electronics are able to monitor the progress, in the form of turns delivered, as well as detect procedural errors, such as magnet decoupling. The force applied by the implanted telescoping rod can be accurately measured by monitoring the current necessary for the external controller to drive the extension of the implanted rod. After characterization, the system was shown to reliably deliver extension distances within 34 μm and maintain coupling out to 70 mm. The system is also able to measure torques as low as 0.12 mN m. System variability and accuracy of external monitoring are addressed.

Copyright © 2012 by American Society of Mechanical Engineers
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Figure 1

(a) X-radiograph of an implanted device in a live ewe for proof of concept and initial biocompatibility testing before adjustment. (b) X-radiograph of an implanted device after adjustment by 2 mm. Implant was fixed to the lumbar vertebrae with a 6 mm spinal fixation rod and an inline connector. The other end of the implant was fixed to the cranial end with the same connectors. The implant was lengthened over the course of 2 weeks in 1 mm increments. Torque measurements were not obtained for initial testing.

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Figure 2

Current prototype of the ROBOImplant controller

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Figure 3

Latest prototype for the ROBOImplant

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Figure 4

Block diagram of the external ROBOImplant control circuitry

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Figure 5

Results of the dc motor characterization. The above plot shows the angular velocity of the Maxon motor versus the current drawn by the motor. The measured data matches well to the expected linear relationship expected between angular velocity and torque for a dc motor.

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Figure 6

Calibration curve for the controller and implant. The extension distance of the implant was recorded as a function of the desired extension programmed into the controller with the separation distance and orientation fixed. Each measurement was repeated five times. The plot above shows that the extension distance of the implant is proportional to number of turns of the outer magnet, as expected. Furthermore, for a given desired extension distance, the variability of the resulting implant extension is less than 34 μm, much smaller than the typical 1 mm dose. From the fitted relationship between extension distance and desired turns, the ratio of the extension distance to turns is approximately 1.3 μm per turn of the external magnet.

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Figure 7

Plot of drive current versus implant and driver magnet. The motor current through the driver motor was measured as a function of the separation distance between the driver magnet and implant magnet. The standard errors of the measurements are shown as error bars. Decoupling occurred at a separation of approximately 70 mm (2.76 in.). As the separation distance between the two magnets is increased, the current drawn by the motor decreases, indicating that the load on the motor decreases. At around 30 mm of separation, the motor current levels off at 114 mA, the minimum amount of current drawn by the driver motor when coupled to the implant magnet. Separation distances larger than 70 mm cause the driver magnet to decouple from the implant magnet, causing the driver motor to run unloaded. At this point, the motor current drops to 109 mA. Of note is the current required by the motor at approximately 10 mm of separation. This large current draw was due to mechanical resonance in the implant, requiring the controller to apply more torque to drive the implant to overcome frictional losses.



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