Self-adjusting Electromechanical Gastric Banding SystemErin Crosby1, Andrew Dickerman1, Joshua Mabasa2, Brian Reis2

Robert J. Roselli2, Thomas P. Rauth3

1Department of Mechanical Engineering, 2Department of Biomedical Engineering, 3Center for Surgical Weight LossVanderbilt University, Nashville, TN, USA

INTRODUCTION

ACKNOWLEDGEMENTSWe would like to thank both our faculty advisor, Dr. Robert Roselli, our Vanderbilt Medical Center advisor, Dr. Rauth, for helping guide and direct our project, as well as some essential empirical data. We also had a significant amount of expert advice and assistance from Dr. Michael Goldfarb. We would also like to thank our corporate sponsor, Chris Miller of Ethicon Endo-Surgery for additional help along the way and the financial backing for our project.

METHODSBackground: Gastric Banding is a minimally invasive bariatric weight loss procedure that involves placing an inflatable gastric band around the upper part of the stomach. A saline solution is used to inflate the band, reducing the stomach size, causing a decrease in appetite.

Adjustments typically begin six to eight weeks after the initial surgery and vary depending on each individual patient.

According to the National Health Accounts, obesity health care costs are currently costing the United States over $47 billion annually. The individual costs of obesity total over $18,000 annually with over $10,000 coming from healthcare costs. The typical cost of the current Lap-Band procedure runs from about $12,000 to $25,000.

Most patients experience an average loss of 50% of their excess weight in the first 1-2 years.

Problem Statement:

There are several problems with the current band. The most important issue is that the band is unable to maintain the desired contact pressure in between adjustments and therefore diminishes in effectiveness during this period. The current adjustment procedure is also invasive. The injection port is located below the skin of the abdomen, which means that patients need to be injected with a needle to make an adjustment. Furthermore, between 7 and 14% of patients experience soft tissue infection, port leakage, and tube cracks.

Objectives: Determine the cause for the band's loss of effectiveness over time.

Maintain a desired a contact pressure between the band and the stomach using an electrical feedback system.

Solve the problems associated with a fluid filled system: soft tissue infection, port leakage, and tube cracks.

Have the band automatically adjust itself to reduce the number of regular post-op corrections.

CONCLUSIONS

By addressing the evident problems associated with the static fluid-filled gastric banding system, we were able to devise a comprehensive, compact self-adjusting electro-mechanical system to alleviate these issues. Through mathematical calculations it we have shown that the conceptual design, as well as specific choice of components for our design, is practical and feasible. Using the 0615_003S motor with the 06/1 gearhead operating on 3.0 V, this system could be run continuously for over 10 hours, translating to over 10 years for weekly adjustments off of two common AAA NiCd batteries.

As the application for a device of this type of device would be implantation in human subjects, there are several additional venues that would have to be explored and issues resolved. The whole device would need to include a biocompatible interface. This would protect both the subject from any harmful interactions between body and device and help the device last longer and maintain effectiveness by avoiding corrosion. Additionally, as the device does initially have to be implanted, and considering that the current banding procedure is undertaken laparoscopically, a means for easily inserting the whole device through a small port in the subject’s abdomen and attaching it accurately around the stomach would be essential. There is additional potential for a device such as this to also function as an indicator of gastroesophageal function. If equipped with additional circuitry and a short-range radio frequency transmitter the device could transmit real-time feedback to medical researchers and give insight and valuable feedback of the functionality of the device.

We believe that the promise of an electromechanical gastric banding system is exceptional and that a continuation of the design concepts presented here is strongly merited.

The approach and process for solving all the problems associated with the current fluid-filled pressurized gastric banding system was to design and create a mechanically-constricting, self-adjusting band. This band would in turn resolve the problems currently experienced with the existing band (port leakage, tube cracks, and soft tissue infection around port) by eliminating the tubing, port, and fluid all-together, uniformly constricting the stomach through electro-mechanical means. This also lends to design simplicity, as the device is to be confined to one comprehensive unit containing the band, motor, force sensors and electrical feedback system.

Design considerations had to next be taken into account. The design needed to be feasibly fabricated, cost effective to present banding systems, require comparable or less device maintenance, and designed with power consumption in mind. Biweekly or fewer self-adjustment readings taken would help limit the amount of power used.

We have designed a feedback system with circuitry to have the band maintain a desired set force on the stomach. Variable resistors that change resistance with applied forces translate into changes in output voltage that would determine if the band needs to constrict, stay the same, or loosen. Frequent self-adjustments will reduce the number of regular post-op corrections.

DEVICE FUNCTIONALITY

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Figure 2. a) Ex-vivo volume, b) Ex-vivo pressure, c) In-vivo volume, d) In-vivo pressure data taken for the Lap-Band system at intervals of 6 weeks.

Problem

As can be seen in Figure 2 (a-b), the Lap-Band system suffers from a slight loss of internal pressure outside of the body, with the internal volume remaining constant. Figure 2 (c-d) shows a dramatic increase in internal pressure loss with only a slight decrease in internal volume. This data would not only indicate that the fluid-filled gastric banding system looses effectiveness between adjustments due to physiological changes In the body (evident comparing ex-vivo to in-vivo data) but that the band itself loses some internal volume between adjustments.

PROTOTYPE DESIGN

a)

Figure 3. A) Band assembly with open gear housing, B) Exploded band-housing assembly, and C) Assembled housing

b) c)

Figure 1.

Lap-Band system: A) Unfilled, B) Half filled, C) Depicted placement on stomach

a) b) c)

Problem and Solution Outlinea) b) c) d)

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Paired T-test: # p = 0.08 , * p = 0.09, + p = 0.83

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Paired T-test: # p < 0.0001 , * p < 0.0001, + p < 0.0001

DESIGN OPTIMIZATIONDesign Considerations:

Band must constrict from 2.5 cm to 1.6 cm inner diameter

Motor with gearhead must exert enough torque to fully tighten band

Minimal power consumption as device will be battery powered

Use of small, low-power motor

Gear down motor to minimize torque requirement

Minimal size for implantation of device to be practical

Linear Force, Fl, required to at position with radius ‘r’, contact

pressure ‘P’ , and band width ‘w’,

Representative values at tightest conditions:

P = 55 kPa, r = 0.8 cm, w = 1.2 cm

Results in: Fl = 52.8 N

wrPFl

Motor torque required to produce sustaining linear force

With a gear spacing, G = 1.5 mm and Fl = 52.8 The required torque from the motor, Tr = 12.8 mNm (Specific motor and gearhead selection output a torque of 20 mNm)

lr FG

2

Component Specifics Motor: MicroMo – 0615_003S

Brushless DC motor

6 mm diam., 15 mm length, 2 g

Stall torque: 0.22 mNm

Gearhead: 06/1

Reduction ratio of 256:1

6 mm diam., 17.3 mm length, 4g

Force Sensor: FlexiForce®

Simple variable resistor

Δr

Fl

P

r

Force/Torque Calculations

Solution

It is the aim of our project to alleviate the problems associated with the fluid-filled system as well as the inherent variability of a static band. Thus design of a band that constricts mechanically and includes an electrical feedback system tied into contact force sensors is proposed. Our specific band design works similarly to a hose clamp, with a band that creates a loop and overlaps itself as a screw tightens. This type of design serves several purposes including the introduction of non-back-drivable gearing so that our band requires no power to maintain its position as well as a gear-down that will result in lower motor torque requirements.