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Hypodermic needles are in widespread use, but patients are unhappy with the pain, anxiety, and difficulty of using them. To increase patient acceptance, smaller needle diameters and lower insertion forces have been shown to reduce the frequency of painful injections. Guided by these observations, fine needles and microneedles have been developed to minimize pain and have found the greatest utility for delivery of vaccines and biopharmaceuticals such as insulin. However, pain reduction must be balanced against limitations of injection depth, volume, and formulations introduced by reduced needle dimensions. In some cases, needle-free delivery methods provide useful alternatives.
Keywords:
drug delivery, hypodermic needle, insulin delivery methods, microneedle injection, needle gauge, needle length, pain from needle insertion
To mitigate pain from hypodermic injections, the effect of needle geometry on pain has been investigated. Needle gauge has been shown to significantly affect the frequency of pain during needle insertion into the skin of human subjects.9 For example, insertion of a 27- or 28-gauge needle () had an approximately 50% chance of being reported as painful, which was significantly greater than insertion of a 31-gauge needle (), which had a 39% chance of causing pain. The likelihood of bleeding was also observed to decrease with decreasing needle diameter. Increasing needle length is also expected to increase pain, although to our knowledge the literature does not contain formal studies specifically demonstrating this effect.
In addition, the mechanics of needle insertion has been found to significantly affect pain. Both the force and the mechanical workload (i.e., area under the force-displacement curve) of hypodermic needle insertion have been found to positively correlate with the frequency of pain.10,11 Thus, needle tip sharpness and other factors, such as lubrication, which can reduce the force of insertion and mechanical workload,12 are important parameters that can be optimized to reduce pain from needle insertions.
Motivated to make less painful needles, there has been growing interest in fabricating smaller needles that should be less painful. Progress in this field has been limited by the need for small needles to reliably insert into the skin, to have sufficient mechanical strength, and to be manufactured in a cost-effective manner.
By scaling down conventional manufacturing processes, a number of companies have developed fine needles smaller than 30 gauge that are used largely for insulin delivery. Examples include 31-gauge Micro Fine Plus® needles (Becton Dickinson, Franklin Lakes, NJ) and 33-gauge NanoPass® needles (Terumo, Tokyo, Japan), both of which measure 5 mm in length ( and ). A significant reduction in pain and bleeding from use of a 33-gauge needle compared to a 31-gauge needle has been demonstrated.13 However, patient satisfaction was improved only after application of a suitable lubricant to the 33-gauge needle, which presumably reduced insertion force and workload.
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To further minimize pain during injection, 31-gauge needles have been manufactured to be just 1–3 mm long (). These needles are further designed to remain within the skin and thereby facilitate intradermal vaccination, which may be facilitated by targeting antigen delivery to the skin's Langerhans and dermal dendritic cells. A number of vaccines have been delivered in this way to animal models and human clinical trials are well under way.14,15 These short needles also have potential for delivery of other therapeutics into the dermis, which is well vascularized16 and can thereby enable rapid uptake of drugs into systemic circulation with improved pharmacokinetics.
Even submillimeter needles can be effective, because the primary barrier to delivery of drugs into the skin is its topmost layer called the stratum corneum, which is just 10–20 μm thick.17 Recognizing this fact, micrometer-scale needles have been developed to deliver drugs into the skin ( –).18–21 These microneedles are sufficiently long to penetrate through the stratum corneum, yet small enough to cause little or no pain. Delivery of insulin using microneedles has been demonstrated in diabetic animal models19–21 and, more recently, in diabetic human subjects.22 Because of their very small size, novel microfabrication methods have been adapted from the microelectronics industry to produce these microneedles using methods suitable for inexpensive mass production.
Confirming the hypothesis that microneedles can avoid pain, a study in human volunteers found that 150-μm-long microneedles were reported as painless.23 More recent results from our laboratory examined the effect of microneedle geometry on pain in greater detail and concluded that microneedle length and the number of microneedles are the most important geometric parameters affecting pain and that 500- to 750-μm-long needles can cause 10 to 20 times less pain than a 26-gauge hypodermic needle (data not shown).
Reducing needle size reduces pain and generally increases patient acceptance. The increasing popularity of the short, 31-gauge pen needle is a notable example.24 However, smaller needles are not suitable for all applications. For example, rapid delivery of large volumes and administration of formulations with large particulates require larger needles. Furthermore, scaling down needle length also prevents injection into deeper tissues. Microneedles are just long enough to deliver into the skin, which may be an advantage in some scenarios, but is a drawback in others. Moreover, as needle size approaches the dimensions of skin surface topography and mechanical deformation, microneedle insertion into the skin becomes more difficult and may, in some cases, require specialized insertion devices.25,26 Thus, there is a trade-off between pain and other delivery considerations when smaller needles are used. The correct balance must be obtained for each application. Based on current literature and applications, delivery of vaccines and protein biotherapeutics appears to be most suitable to benefit from the use of smaller needles.
In some cases, the limitations of hypodermic needles can be addressed by eliminating the needle altogether. However, these needle-free alternatives each have limitations of their own. For example, jet injectors accelerate liquid droplets across the skin at high velocity and are used clinically to administer insulin, vaccines, and other drugs, but have had limited impact because of their size, cost, and inability to reduce pain and injury.27 Transdermal patches have also been developed to passively deliver drugs across the skin, but this approach has been limited to hydrophobic and small molecules.17 Skin pretreatment methods such as ultrasound, electric fields, solid microneedles, and thermal ablation are being investigated to increase the permeability of skin for protein and vaccine patches.17,28 New approaches, such as pulmonary, oral, and nasal delivery routes, are increasingly being studied for the systemic delivery of compounds that currently require injections.29 Notably, pulmonary delivery of insulin (Exubera®, Pfizer, Groton, CT) is already approved by the Food and Drug Administration.30
In conclusion, smaller needles can reduce pain and provide other advantages that can increase patient compliance. Fine needles of 33–31 gauge have already gained clinical acceptance and still smaller microneedles are under development. However, smaller needles are not suitable for all applications and, in some cases, needle-free delivery systems provide useful alternatives.
We are thankful to Dr. Young Bin Choy and Jeong Woo Lee at Georgia Tech, Dr. Kyuzi Kamoi at Nagaoka Red Cross Hospital, and Dr. John Mikszta at BD Technologies for providing images of the needles shown in . This work was supported in part by the National Institutes of Health. Mark Prausnitz is the Emerson-Lewis Faculty Fellow at Georgia Tech.
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