Research Papers

Inductive Sensing to Detect Tissue Thickness Between Magnets for Potential Application in Magnetic Compression Based Anastomosis

[+] Author and Article Information
Anupama Arun

Department of Surgery,
University of California San Francisco,
San Francisco, CA 94143;
Department of Bioengineering and
Therapeutic Sciences,
University of California San Francisco,
San Francisco, CA 94143
e-mail: anupama.arun@ucsf.edu

Brandon Gaston, Dillon Kwiat, Jill Imamura-Ching, Richard Fechter, Michael Harrison

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

Shijie Chen, Hao Jiang

School of Electrical Engineering,
San Francisco State University,
San Francisco, CA 94132

Shuvo Roy

Department of Bioengineering and
Therapeutic Sciences,
University of California San Francisco,
San Francisco, CA 94143

Manuscript received June 20, 2015; final manuscript received December 22, 2015; published online February 17, 2016. Assoc. Editor: Rosaire Mongrain.

J. Med. Devices 10(1), 011008 (Feb 17, 2016) (7 pages) Paper No: MED-15-1204; doi: 10.1115/1.4032615 History: Received June 20, 2015; Revised December 22, 2015

Magnetic compression based anastomoses use magnetic force to necrose tissue between two magnets to create an anastomosis. Nickel-plated neodymium–iron–boron magnets are used in our study. The compression pressure between the magnets depends on the distance between the magnets, which is determined by the thickness of the compressed tissue and depends on bowel wall thickness and elasticity. It is critical to know the distance between the magnets once the tissue is compressed because the magnets must be within a critical distance of each other in order to create enough compressive force to necrose the tissue. We have developed an inductance sensor to detect the distance (tissue thickness) between the two magnets after the surgeon has deployed them. Inductance sensing is a contact-less sensing method that enables precise short-range detection of conducting surfaces. The inductor coil mounted on one magnet detects the second magnet by measuring the change in inductance due to eddy current induced on the nickel-plated surface of the second magnet. The change in the inductance is proportional to the change in distance between the magnets. The sensor was first calibrated by using polycarbonate sheets to simulate the intestine tissue. We are able to detect up to 6 mm of spacing between the magnets. Pig intestine from Yorkshire pigs was used to characterize the sensor. We are able to distinguish up to five distinct layers of the intestine from the large intestine. This sensing mechanism can indicate the operating surgeon the exact thickness of the tissue compressed between the two magnets. The surgeon can thus be sure of formation of a clean anastomosis and avoid the likelihood of the magnets sliding away or uncoupling.

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Fig. 1

Illustration of the Magnamosis procedure: (a) the first magnet is installed at the distal end using laparotomy and (b) the second magnet is installed at the proximal end using an endoscope. In this case, it is in the stomach. (c) The two magnets are bought close to each other. (d) The magnets are self-aligned and are attracted to each other. (e) Once the entire tissue between the two magnets is completely necrosed, it is dislodged and comes out of the rectum.

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Fig. 2

Orientation of the inductor coil relative to the magnet

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Fig. 3

Theoretical estimation of the inductance of a spiral inductor coil as function of number of turns

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Fig. 4

S-parameter measurements of the custom-made inductor coil connected as a tank circuit with capacitor in parallel: (a) modulus and phase of s11 and (b) modulus and phase of s12

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Fig. 6

Testing configuration. The inductor coil printed on a printed circuit board is placed on the first magnet. Polycarbonate sheets of thickness 0.6 mm are used to simulate the tissue. (a) The second magnet is not modified and is used as is and placed on the polycarbonate sheets. (b) The aluminum foil is used as the second conducting target. (c) The aluminum foil is used in conjunction with the second magnet. (d) Symmetric configuration. Two coils are mounted on each magnet. The two coils are connected in series.

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Fig. 7

Experiment setup with pig intestine between the two magnets. The bottom magnet and the inductor coil are kept in a polycarbonate cover to avoid short circuiting of the inductor coil due to contact with the tissue. In a clinical setting, this will be avoided be including the inductor coil within the polycarbonate casing that holds the magnet.

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Fig. 8

Inductance change between the two magnets as a function of polycarbonate thickness. Nickel-plated magnet refers to the configuration in setup (Fig. 6(a)); aluminum foil below the second magnet refers to the configuration in Fig. 6(c); and aluminum foil only refers to the configuration shown in Fig. 6(b)

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Fig. 9

Change in inductance for a symmetric setup with two inductor coils in series. One coil is mounted below the magnet, and the second magnet is on top of the magnet. Both the magnets are thus identical and is referred to as symmetric setup/also shown in the same graph the results with single coil to highlight the contrast.

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Fig. 10

Inductance change for different thicknesses of pig intestine. Experimental setup used to measure the thickness of pig intestine is shown in Fig. 7. The thickness of the tissue ranged from 0.45 mm to 0.75 mm.




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