(482g) Effect of Microneedle Design on Pain in Human Subjects

INTRODUCTION Oral drug delivery is associated with problems like drug interaction with food, ?first-pass' drug degradation in the liver, and the enzymatic drug degradation in the gastrointestinal tract. To overcome these problems skin has been extensively studied as an an alternative route of drug delivery. Skin is a large and easily accessible organ that can be readily used to administer drugs into the blood capillaries lying just tens of microns beneath the skin's surface. Despite the advantages offered by skin for drug delivery, clinical drug delivery through the skin is severely limited by the presence of the top most layer of dead cells called the stratum corneum. This layer is just 10-20 µm in depth, but is the rate limiting barrier and only allows low molecular weight molecules with moderate oil and water solubility to diffuse through. This in turn restricts the drugs that can be delivered via the skin into a very narrow range. As a result, presently only thirteen active molecules are approved for delivery through the skin by the Food and Drug Administration.

To increase the repertoire of drugs deliverable via the skin in a painless manner, and with increased safety over hypodermic needles, microneedles were developed. Microneedles are micron-dimensioned needles that can pierce the skin. Upon piercing skin they create micro-conduits across stratum corneum and provide a direct route for drugs and vaccines into the skin. Microneedles have been shown to deliver drugs and vaccines into the skin in vitro and in vivo, including administration of insulin to diabetic animals and elicitation of potent immune responses to transdermal vaccines. One of the advantages claimed for delivery using microneedles is that their microscopic size enables them to be painless. However, until now only one study has quantitatively measured pain in human subjects caused by insertion of microneedles. This study used just one microneedle design involving very short (150 µm) microneedles and found sensation caused by these microneedles to be insignificant compared to insertion of a conventional hypodermic needle. As part of another study, blunt-tipped ?microenhancer arrays' having lengths of up to 200 µm were shown to cause weak to very mild perception in humans. These ?microenhancer arrays' were, however, rubbed on the skin as opposed to inserted into the skin.

Since these initial studies, microneedles of larger dimensions (up to 1000 µm) have been fabricated and used. Bigger microneedles can be easier to fabricate, are generally easier to insert, and provide enhanced drug delivery capabilities. But these larger microneedles still need to avoid pain. To address the trade-off between microneedle function and pain, the goal of this study was to determine the effect of microneedle geometry on pain felt during insertion into the skin of human subjects. Guided by these results, microneedle geometry can be designed to produce optimal microneedles that effectively deliver drug and still avoid pain.

EXPERIMENTAL METHODS Single microneedles with different geometries were fabricated using an infrared laser to cut microneedles from 75 µm-thick, stainless steel sheets. To deburr and sharpen the microneedle edges, electropolishing was done in a 1:3:6 v/v mixture of water: phosphoric acid: glycerine at 70°C. To study the effect of microneedle length on pain, needle length was varied over 500, 750, 1000 and 1500 µm with a constant tip angle of 55 degrees. The effect of tip angle on pain was analyzed by varying tip angles over 20, 55 and 90 degrees at two different lengths of 500 and 1000 µm each. Insertion of a 5 mm-long, 26 gage hypodermic needle into the skin was used as a positive control. As a negative control, a flat end of a 0.125 inch diameter teflon rod was pressed against the skin surface. Ten human subjects were recruited for the study according to the protocol approved by the Institutional Review Boards of Emory University and Georgia Institute of Technology. Each subject received microneedle insertions using the different needle geometries, a positive control and a negative control, all in triplicate by manual insertion into the subject's forearm. The subjects were kept blinded to the treatments and were asked to quantify the pain after each treatment on a Visual Analog Scale.

To analyze the data, triplicate pain scores for each treatment on a given subject were averaged and then normalized with respect to the average pain score of that subject's positive control. This normalization relative to a hypodermic needle helped address the subjectiveness of pain between subjects by using each subject as his or her own control.

RESULTS AND DISCUSSION Our interest was to determine the effects of microneedle length, width, thickness, tip angle, and number of microneedles on pain in human subjects. Stainless steel flat microneedles can be easily fabricated in a variety of shapes and sizes, and can also be made in the form of arrays, making them an ideal candidate for this study.

The effect of microneedle length was studied by measuring the pain elicited by microneedles ranging in length between 500 ? 1500 µm, at a constant tip angle of 55 degrees. The subjects reported minimal pain for 500 µm-long microneedles, which was equal to just 5% of the sensation caused by a hypodermic needle. As microneedle size was increased, pain also increased with increasing needle length up to a value equal to 35% of the sensation caused by a hypodermic needle for 1500 µm-long microneedles. These data suggest that microneedle length is an important parameter that has a strong effect on perceived pain (ANOVA, p<0.05) The effect of tip angle was also studied. Tip angle was found not to have a significant effect on pain during microneedle insertion (ANOVA, p<0.05). Sensation caused by microneedles having three different tip angles ? 20º, 55º and 90º ? were found to be statistically indistinguishable using needles of 500 µm and 1000 µm length. We expected that tip angle, which is a measure of tip sharpness, would have an effect on pain because sharper needles require less force to insert into the skin. However, our result was not consistent with this expectation. This inconsistency may be explained by considering that the microneedles used in this study were just 50 µm thick. It may be that with such thin needles, changing needle tip geometry did not significantly affect the insertion force.

The subjects generally reported that the microneedle insertions did not cause discomfort, especially for the shorter needles. Sometimes the subjects described the sensations during microneedle insertion as hot, cold or burning. These sensations can be expected to occur since pain nociceptors (unmyleinated C fiber) are also responsive to thermal stimuli. Visual observation of the microneedle insertion sites revealed no skin irritation within minutes after treatment. The subjects reported no adverse events.

In addition, a series of studies examining the effect of width, thickness and number of microneedles on pain are currently on-going in our laboratory. These studies will be completed soon and reported on at the conference.

CONCLUSION Stainless steel microneedles covering a range of different geometries were fabricated using laser cutting and electropolishing. Analysis of pain scores reported by human subjects after insertion of single microneedles showed that pain increased with increasing microneedle length, but had no statistical dependence on tip angle. The optimal microneedle design produced by this study suggests that microneedles with a length of 500 ? 750 µm and a width of 50 µm cause just 5 ? 10% of the sensation produced by a 26 gage hypodermic needle and, according to other studies, are effective for drug delivery.

ACKNOWLEDGEMENTS This work was supported in part by the National Institutes of Health and took place in the Center for Drug Design, Development and Delivery and the Microelectronics Research Center at Georgia Tech.