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Research Papers

Effects of Microneedle Design Parameters on Hydraulic Resistance

[+] Author and Article Information
R. Lyle Hood

School of Biomedical Engineering and Sciences,  Virginia Tech, Blacksburg, VA 24061

Mehmet A. Kosoglu, Matthew Parker

Department of Mechanical Engineering,  Virginia Tech, Blacksburg, VA 24061

Christopher G. Rylander

School of Biomedical Engineering and Sciences and Department of Mechanical Engineering,  Virginia Tech, Blacksburg, VA 24061

J. Med. Devices 5(3), 031012 (Sep 06, 2011) (5 pages) doi:10.1115/1.4004833 History: Received December 22, 2010; Revised August 01, 2011; Published September 06, 2011; Online September 06, 2011

Microneedles have been an expanding medical technology in recent years due to their ability to penetrate tissue and deliver therapy with minimal invasiveness and patient discomfort. Variations in design have allowed for enhanced fluid delivery, biopsy collection, and the measurement of electric potentials. Our novel microneedle design attempts to combine many of these functions into a single length of silica tubing capable of both light and fluid delivery terminating in a sharp tip of less than 100 μm in diameter. This paper focuses on the fluid flow aspects of the design, characterizing the contributions to hydraulic resistance from the geometric parameters of the microneedles. Experiments consisted of measuring the volumetric flow rate of de-ionized water at set pressures (ranging from 69 to 621 kPa) through a relevant range of tubing lengths, needle lengths, and needle tip diameters. Data analysis showed that the silica tubing (∼150 μm bore diameter) adhered to within ±5% of the theoretical prediction by Poiseuille’s Law describing laminar internal pipe flow at Reynolds numbers less than 700. High hydraulic resistance within the microneedles correlated with decreasing tip diameter. The hydraulic resistance offered by the silica tubing preceding the microneedle taper was approximately 1–2 orders of magnitude less per unit length, but remained the dominating resistance in most experiments as the tubing length was > 30 mm. These findings will be incorporated into future design permutations to produce a microneedle capable of both efficient fluid transfer and light delivery.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 2

Hydraulic circuit analytical model representation of resistance in (a) a capillary tube (stage I) and (b) tube with a microneedle tip (stage II)

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Figure 3

The left image shows the fluid deployment device (FDD) with specific components labeled. The right image shows a schematic of the microneedle coupling. The microneedle is epoxy bonded into an 18 gauge syringe needle, which couples to a Luer-Lok to 1/4 in. NPT adaptor at the distal end of the FDD.

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Figure 4

Graph of pressure versus volumetric flow rate for different lengths of straight tubing in stage I. Both experimental data and theoretical values predicted by Poiseuille’s Law are shown. Experiments were conducted at N = 5 for each length of tubing.

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Figure 5

Measured flow rate versus pressure for tubes and microneedles. Error bars are included on all points, but do not extend outside the marker in some cases. Experiments were conducted with N = 5 for each microneedle.

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Figure 6

Hydraulic resistance of the 14 microneedles plotted against their tip diameters

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Figure 1

(Left) Three sample hollow-core microneedles. The scale bar represents 500 μm. Water within the needle’s bore can be seen in the image of microneedle 8. (Right) En face images of the tips of (a) flat-cleaved tubing, (b) needle 10, (c) needle 11, and (d) needle 113. Scale bar represents 100 μm.

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