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Research Papers

An Electronic Force Sensor for Medical Jet Injection

[+] Author and Article Information
Nickolas P. Demas

BioInstrumentation Laboratory,
Department of Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: demas@alum.mit.edu

Ian W. Hunter

George N. Hatsopoulos
Professor in Thermodynamics,
BioInstrumentation Laboratory,
Department of Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: ihunter@mit.edu

1Corresponding author.

Manuscript received July 11, 2018; final manuscript received February 28, 2019; published online April 11, 2019. Assoc. Editor: Michael Eggen.

J. Med. Devices 13(2), 021007 (Apr 11, 2019) (8 pages) Paper No: MED-18-1113; doi: 10.1115/1.4043196 History: Received July 11, 2018; Revised February 28, 2019

In medical jet injection, a narrow fluid drug stream is propelled at high velocity into skin without a needle. Previous studies have shown that the volume delivered is highly dependent on a number of factors. This paper details the development of an electronic force sensor for medical jet injection and shows that the normal contact force exerted on the tissue by the nozzle is an additional factor affecting volume delivered. Using this sensor, we measure the forces at the nozzle tip in the normal direction with a sensitivity of 18 μN, calibrated over a range from 1 N to 8 N with a mean absolute error of 8 mN, and a maximum overload of 300 N. We further measure forces at the nozzle tip in the lateral direction with a sensitivity of 8 μN, calibrated over a range from 0.1 N to 7 N, with a mean absolute error of 101 mN for lateral contact force magnitude and 1.60 deg for lateral contact force direction. Experimental validation confirms that the force sensor does not adversely affect the accuracy and precision of ejected volume from the jet injector. We use this setup to examine the effect of normal contact force on volume delivered into postmortem porcine tissue. Experimental results demonstrate that volume delivered with normal contact force between 4 N and 8 N is significantly more accurate and precise compared to volume delivered with normal contact force between 0 N and 3.9 N.

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References

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Shergold, O. A. , Fleck, N. A. , and King, T. S. , 2006, “ The Penetration of a Soft Solid by a Liquid Jet, With Application to the Administration of a Needle-Free Injection,” J. Biomech., 39(14), pp. 2593–2602. [CrossRef] [PubMed]
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Figures

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Fig. 1

Possible contact force configurations showing the skin, ampoule, force sensor, and jet injector. (a) shows a desired loading condition where normal force is applied at a desired level and lateral forces (Fx and Fy) are minimized. Also shown are nonideal loading conditions including (b) insufficient normal force, (c) excessive normal force, and (d) excessive lateral force (in this case along the y-axis).

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Fig. 2

Assembled force sensor alone without adapter, ampoule, or housing with the strain relief PCB mounted on top

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Fig. 3

Flexural beam mechanical design shown without strain gauges or strain relief PCB. Note the notch cutout at the inside edge of the flexures.

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Fig. 4

Detail of electrical connections made with magnet wire between a dual-bridge strain gauge and the strain relief PCB

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Fig. 5

Cutaway view of force sensor mounted on adapter with ampoule and housing

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Fig. 6

Force sensor mounted on the pre-existing jet injector at top [1]. Illustrations of the three-flexure sensor (with the location of each flexural beam n noted) and the alignment with Fnormal, Fx, and Fy with Θ equal to zero are shown at bottom with a left side view (left bottom) and a front view (right bottom).

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Fig. 7

The normal contact force calibration jig being used to calibrate normal contact force. The jig top rests on the nozzle of an upright ampoule mounted in the injector. Monofilament line connects the calibration weight to the jig top, applying a normal contact force.

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Fig. 8

Measured bridge voltage versus flexural beam load. The error bars, which indicate one standard deviation above and below the mean, are obscured behind the data points. n = 100 for each discrete force level. The reported equation (and R2 value) is for the line shown, fit using least squares.

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Fig. 9

Measured normal contact force versus applied normal contact force. Dotted line indicates perfect agreement between applied and measured values. Shaded regions show where ±0.5 N is exceeded. The unshaded region conforms to the specification. All measurements fall in the unshaded region and conform to the specification. The error bars indicate one standard deviation above and below the mean. n = 100 for each discrete force level.

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Fig. 10

The lateral contact force calibration jig being used to calibrate lateral contact force magnitude and direction. The monofilament line extends perpendicularly from the vertically mounted nozzle so that no normal contact force was exerted on the nozzle.

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Fig. 11

Experimentally measured lateral forces fall very close to the expected location at the intersection between the radial lines (representing applied angles) and circles (representing applied magnitudes)

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Fig. 12

FEA displacement results due to 300 N static axial load. Deformation is exaggerated for visualization purposes (200×).

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Fig. 13

FEA von Mises stress results due to 300 N static axial load. Plastic deformation would not occur as the yield stress is 503 MPa for 7075-T6 aluminum alloy [19]. Deformation is exaggerated for visualization purposes (200×).

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Fig. 14

These plots compare the results from the ejection validation experiments conducted with the pre-existing jet injector [1] for various volumes and jet velocities. The black dashed line shows the target volume. Shown in gray boxes are results with the jet injector without force sensor. The white boxes show the volume ejection performance with the force sensor. For all tests, n = 30. The error bars indicate one standard deviation above and below the mean.

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Fig. 15

The top of the porcine tissue block after being injected with water mixed with tissue-marking dye at an ejection volume of 100 μL. The injection locations are visible as red dots (resulting from the injectate penetrating the tissue) in a regular grid pattern.

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Fig. 16

The relationship between volume delivered and normal contact force for 100 μL injections into porcine tissue using a vjet of 200 m/s, tjet of 10 ms, and vfollow of 50 m/s at a low and high normal contact force ranges

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Fig. 17

Rotation correction schematic showing how the sensor's axes are aligned with the user's axes using Eq. (A1). Presuming the user's axes are oriented like a traditional Cartesian coordinate system for the reader, the sensor is mounted at a rotation of −θ due to geometric requirements of the jet injector. A rotation about the origin by angle θ is required to align the sensor's axes with the user's axes.

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Fig. 18

Skew correction schematic indicating how the sensor's lateral axes are orthogonalized using Eq. (A2). The input axis system is skewed by angle parameters a and b, which relate the original and desired y-and x-axes, respectively.

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