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Pain and Porcupines

While hypodermic needles come in a dizzying array of options in terms of sizes and point styles, ultimately not much has changed since they hit the scene over a century ago. Many of us are lucky enough not to require too many injections in our lifetime but, for some, the pain of using a needle is a daily healthcare burden – making compliance with injected therapies a real challenge.

It was the discovery of insulin in 1921, and the realisation it required injection into the bloodstream, that created the market for hypodermic needles. Fast forward to today, and the modern needle has remained similar to its predecessors but is now available in a wide range of diameters and lengths, as well as tip designs for different applications: single or multi-bevel facet, reverse bevel, spoon point, pencil or diamond point, trocar, and then the 'non-coring' designs such as the Huber-point needle.

Of chief concern to most people getting an injection – apart from the obligatory anxieties over sterility and cleanliness – is ‘how much is this going to hurt?’ For some, the pain associated with being pierced by a needle is a daily trauma, whether they are injecting themselves, or a dependent: an unwilling, uncooperative and distressed child, for instance.

One avenue to ridding individuals of needle pain is to remove the requirement for a needle entirely and use a different delivery mechanism, such as pulmonary, oral or nasal delivery. However, for many drugs, alternate routes of administration are not an option, and eliminating the needle does not necessarily remove the pain – needle-free injections are not known for being gentle.

So, assuming we are stuck with the needle, how can the pain be lessened? There is evidence that lower insertion forces reduce the likelihood of pain, so that would appear to be a good place to start (1,2).

POINT OF INSERTION

The force of a needle consists of three elements:
  • Puncture force – required to initially puncture the skin
  • Cutting force acting on the tip of the needle
  • Friction force acting on the needle’s shaft
After the puncture – as the needle is inserted – both the cutting and friction forces are in play; when the needle is retracted, only the friction force is felt.

As the needle is pushed in, the skin initially moves with the tip of the needle, without being penetrated. The further the skin is depressed, the higher the force it exerts back on the needle tip – until that force is high enough for the needle to puncture the skin. The puncturing can be described by fracture mechanics, with a crack being formed in the skin; the shape of that crack has been shown to depend upon the shape of the needle’s tip (3). For example, a sharp bevel or conical needle forms a planar crack, a diamond tip forms a star-shaped crack, and a blunt or very large bevel angle tip forms a ring crack, like a hole-punch.

The tip insertion phase follows when the crack in the skin is enlarged, and the sharp edges of the tip wedge the cut open. The crack can grow gradually, following a stable ‘cutting’ mode, or can be sudden and unstable, depending on the type of tissue, its properties and how much tension has been built up. As the tip goes deeper and prises the tissue open wider, the insertion force increases.

With the shaft entering the skin, the needle tip continues cutting as it moves deeper but, no longer widening the hole, the force required to cut remains constant. However, as more of the needle is inserted, the friction between the tissue and the shaft increases, as there is more shaft-tissue contact.

DESIGN INFLUENCE

How does the design of the needle affect these forces? In his comprehensive literature review of needle-tissue interaction, van Gerwen noted that, although many studies present insertion force data, dedicated experiments with true relevance and statistically powerful results are thin on the ground (4). Nevertheless, there are some clear indications as to what is going on and what factors might influence the insertion force.

Two design factors clearly have an effect: the shape of the tip (multi-faceted bevel tips with shallow bevel angles exhibit the lowest insertion forces), and the diameter of the needle (smaller sizes require less force). This implies that, for minimal pain, the tip design must be right and the needle as small as possible.The prevalence of very fine insulin needles and the vast research efforts dedicated to microneedles and microneedle arrays – fine, short and reported to be practically painless – are testament to this approach.

But not everything can be delivered with a tiny needle. Rapid delivery of large volumes requires large needles, and intramuscular delivery necessitates needles of sufficient length, strength and size.

INSPIRATION FROM NATURE

It has long been known that porcupine quills are difficult to get out once they have been put in. Building on this, researchers at Harvard and MIT discovered in 2012 that the quills of the North American porcupine go in much more easily than they should (5). The reason for this is that, unlike the African porcupine and hedgehogs which have smooth quills, North American porcupines have microscopic backward-facing barbs running along their quills.

This research discovered that the barbed quills required 54% less penetration force than equivalent barbless quills, and 60% less work to penetrate muscle tissue than a similar sized hypodermic needle. It also revealed that the barbed quills caused less tissue damage than barbless quills. Taking things one step further, the researchers fabricated a prototype hypodermic needle with microscopic barbs, and found that it had a penetration force 80% less than a barbless equivalent.

The explanation given for this outcome was that the barbs acted in the same way as the serrations on a knife, providing stress concentrations near the barbs which cause the tissue to fail locally. This diminishes the need to deform the entire circumference of tissue surrounding the quill and, as a consequence, reduces the penetration force. Such an approach aligns well with previous studies, which highlighted that the increased number of cutting edges found on a multi-faceted bevelled hypodermic needle reduces the penetration force, compared to a single-facet bevelled needle.

PULL-OUT FORCE

Although reduced penetration force and tissue damage should mean a less painful insertion, there is a downside. As one might expect, the force to pull out the barbed quills was about four times higher than for a barbless quill. There could be uses for this in medical applications where tissue adhesion is desirable – for example, in a drug delivery patch featuring an array of needles that you want to keep in place, or in a cannula which, once in, should not be easy to remove. For a standard injection, however, it is preferential to be able to withdraw the needle as easily as possible.

As the research found, it was the flexibility of the barbs on the porcupine quills that provided most of the grip. When the quill was pulled out, the barbs would flex outward – expanding the apparent diameter, increasing frictional resistance and promoting tissue interlocking. In comparison, polyurethane quill mimics had barbs that could not flex, and skin tests showed the pull-out force for the mimic was almost a third of that for a natural quill. Unfortunately, this would still make the pull-out force significantly higher than for a standard hypodermic needle.

OSCILLATION AND DISTRACTION


If we cannot reduce the pain by redesigning the needle, there may be another approach: control over the way in which the needle is inserted. Opportunities may well open up as auto-injectors and pen-injectors become more prevalent, and with the potential control offered by the electromechanical injector.

One method is the oscillating needle, and a case study is illustrative. Actuated Medical’s GentleSharp blood sampling system provides low frequency axial micro-oscillations to the needle tip, achieving peak velocities of greater than 200mm/s. It is based on research which indicates that vibrating needles during insertion lead to reductions in the puncture and friction forces – and subsequently pain – due in part to the viscoelastic properties of tissue. The device has been designed for use with animals; researchers claim that a force reduction into cadaver rat tails of up to 72.6% substantially curtails levels of stress and behavioural responses in rat models. At the moment, the technology has not been determined to be safe and effective for human use, although the company is currently pursuing testing to support a regulatory submission for a human use version.

As well as the oscillating needle creating a lower insertion force, GentleSharp attributes the potential anaesthetising effect of vibrations to the highly influential Gate Control Theory – an effect that other device manufacturers are also keen to exploit. First proposed in 1965 by Melzack and Wall, this theory asserts that a non-painful stimulation, applied simultaneously with a painful stimulation, can prevent the pain signal from travelling to the central nervous system. In other words, the non-painful input closes the gate to the painful input.

OTHER STUDIES

The effectiveness of vibrating injection devices for pain reduction has been disputed (6). However, a study presented at the Anaesthesiology 2014 annual meeting found that the perception of pain was significantly reduced when a specific amount of pressure and vibration was applied prior to a simulated needle stick, and that the application of heat also had a small but not insignificant benefit (7). The authors of this study explained that the pain-reduction effect is likely due to distraction, as well as Gate Control Theory.

Another route to reducing the needle insertion force – and thus the pain – may be for an injection device to actively rotate the needle. In tests with animal tissues, rotating the needle as it is inserted appears to reduce the required insertion force (3). Pulling it out is a different story: there is a phenomenon in acupuncture called ‘needle grasp’, where it feels like the needle has been gripped by the skin. One study discovered that in human skin, rotating the acupuncture needle before pulling it out could actually increase the pull-out force by up to 150% (8). So, any device that rotates the needle on the way in should keep it still on the way out.

Although there are probable limitations to how much we can reduce the pain of injection by looking at the needle alone, as devices that control how the needle is inserted become more commonplace – particularly with electro-mechanical devices – there could well be opportunities to transform our attitudes toward injections. Until then, distract yourself from your jab by thinking about bald porcupines.

References
  1. Egekvist H, Bjerring P and Arendt-Nielsen L, Pain and mechanical injury of human skin following needle insertions, European Journal of Pain 3(1): pp41-49, 1999
  2. Egekvist H et al, Regional variations in pain to controlled mechanical skin traumas from automatic needle insertions and relations to ultrasonography, Skin Research & Technology 5: pp247-254, 1999
  3. Shergold O and Fleck N, Experimental investigation into the deep penetration of soft solids by sharp and blunt punches, with application to the piercing of skin, Journal of Biomechanical Engineering 127(5): pp838-848, 2005
  4. van Gerwen D, Dankelman J and van den Dobbelsteen J, Needle-tissue interaction forces – a survey of experimental data, Medical Engineering and Physics 34(6): pp665-680, 2012
  5. Cho WK et al, Microstructured barbs on the North American porcupine quill enable easy tissue penetration and difficult removal, Proceedings of the National Academy of Sciences 109(52): pp21,289-21,294, 2012
  6. Saijo M, Ito E, Ichinohe T and Kaneko Y, Lack of pain reduction by a vibrating local anesthetic attachment: A pilot study, Anesthesia Progress 52: pp62-64, 2005
  7. American Society of Anesthesiologists, An end to needle phobia: Device could make painless injections possible, Science Daily, 13 October 2014
  8. Langevin H, Churchill D, Fox J, Badger G, Garra B and Krag M, Biomechanical response to acupuncture needling in humans, Journal of Applied Physiology 91: pp2,471-2,478, 2001

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As well as managing projects and expanding Team Consulting’s client base, Philip Canner is responsible for the generation and detailed development of a wide range of medical devices from concept, through prototyping and testing, to production.
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Philip Canner at Team Consulting
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