Biological puncture
and the dissolving patch

A patch has to do more than enter skin

A needle can look successful in a microscope image and still fail as a delivery device. It may cross the skin with only a small part of its payload. It may leave material on the surface, soften before entry or lose biological activity during fabrication. An attractive puncture mechanism solves only part of this problem.

A dissolving vaccine patch must carry a defined formulation, survive manufacture and storage, withstand application, release material into an intended tissue region and leave a manageable used product. Its mechanical properties change during use. The dry structure must resist failure; after application, dissolution must proceed far enough to release the dose within the intended wear time.

A feeding apparatus, a defensive quill and an egg-laying organ operate under different constraints. A feature that improves one may be unnecessary—or troublesome—in another. A quill that is difficult to remove has achieved a defensive purpose. A vaccine patch requires enough contact to deliver its payload, followed by predictable removal of the backing. Those objectives can favour different structures.

Our comparison therefore asks three questions of each mechanism: what was measured, what equipment or material made it possible, and what would have to be demonstrated in a dissolving array? We use selected primary experiments, rather than treating the number of species with sharp structures as evidence for a particular design. The comparison concerns the published mechanisms and their constraints; it does not specify an Alterspan device.

What the mechanisms require

Support and motion: the mosquito and the wasp

A mosquito-inspired biopsy study by Li and colleagues (2020) tested movement between parts as well as needle shape. It compared direct insertion with alternating needle–cannula motion, then added notches to the needle tip. Mean forward speed was kept the same. The experiments tracked displacement in transparent, particle-filled phantoms.

In one phantom path, final displacement was 2.2 mm for direct insertion, 1.8 mm with alternating motion and 1.5 mm with the notched, alternating system. This is evidence about that biopsy arrangement, not a prediction for a skin patch. The notch advantage in the local displacement measurement also diminished near the end of insertion. The same geometry did not provide a constant benefit throughout the motion.

01 · Mosquito-inspired insertion

Watch the material move around the needle.

Read across: direct insertion (NCD), alternating needle and cannula (NCI), then alternating motion with a notched tip (MPI).

Read down: three moments during insertion. Blue marks greater displacement; red marks less. The lower plots show that the advantage changes during the motion.

See the needle design and biopsy set-up

The study translates alternating movement into a biopsy needle and cannula. The medical images illustrate why displacement matters when approaching a target. Original Fig. 1 ↗, reproduced unchanged under CC BY 4.0.

Alternating motion depends on an assembly of moving parts. A conventional one-piece mould does not reproduce that assembly. Removing the motion while retaining the notch changes the mechanism being compared.

The wasp-inspired device developed by Bloemberg and colleagues (2024) makes this dependency visible. Six steel segments advance in sequence, with one moving forward while the others retract. A manual mechanism coordinates them; guides support the slender segments before entry. Its assembled diameter was approximately one millimetre. This was an instrument for advancement through a substrate, tested in gelatin, rather than a dissolving array carrying vaccine.

02 · Wasp-inspired motion

The mechanism includes everything behind the tip.

The instrument shows why support matters alongside tip shape. The unsupported length, the guides and the drive are part of the reported device; the needle tip alone does not explain its performance.

Barbs: entry and retention need separate tests

In Cho and colleagues’ porcupine-quill experiments (2012), barbed polyurethane replicas required less force to enter muscle than smooth replicas: 0.030 N versus 0.046 N at the tested depth, with five samples contributing to each mean. Surface features could concentrate deformation locally. The work also examined resistance to withdrawal. Lower entry force and stronger retention were distinct outcomes of the geometry.

03 · Porcupine-inspired surfaces

Less force to enter. A separate question on removal.

A dissolving array adds several conditions. A small barb must fill in the mould, survive removal from it and remain intact until application. It must still perform after formulation changes alter stiffness and after storage changes moisture content. Once wet, it may soften or dissolve before it provides useful retention. If it persists, its effect on removal and local injury matters.

For a dissolving array, surface features introduce a trade-off between mechanical behaviour, mould filling and release. The quill study establishes an effect in its tested material and tissue. It does not establish the same effect after the feature is transferred to a dissolving formulation.

Swelling: a material can change its function after entry

Yang and colleagues (2013) used a different strategy, inspired by a parasite’s anchoring structure. Their microneedles combined a water-responsive outer material with a non-swelling polystyrene core. The tips expanded after entry and increased attachment to pig skin. The device was designed as a tissue adhesive. It offers a clear example of timing material behaviour, but its supporting core is not a dissolving vaccine matrix.

04 · Swelling after entry

The tip swells after it enters tissue.

The study illustrates a change in function after entry: the structure first supports insertion, then hydrates and expands. A dissolving patch has a different intended endpoint. Swelling, dissolution and release cannot be treated as interchangeable processes.

The useful duration of attachment depends on the device’s purpose. Securing a graft and delivering a vaccine over a brief wear period impose different requirements.

Grooves: liquid delivery is a different architecture

Bae and colleagues’ snake-fang-inspired patch (2019) uses open grooves to convey a liquid formulation through capillary action. The accessible primary abstract supports that operating principle; we did not obtain the full methods for a detailed numerical comparison. It belongs in this analysis because it exposes a boundary: a fluid path is useful only while there is fluid to transport and a surface that supports the required flow.

A groove moulded into a solid dissolving needle does not inherit liquid-delivery performance. It changes the available structural cross-section and may affect wetting and dissolution, but those are new hypotheses. Retaining a liquid formulation could be a worthwhile alternative product architecture. It also brings a reservoir, containment and stability problem that a dry patch is intended to address. That choice should be explicit before borrowing the shape.

Geometry sets a dose constraint

It is tempting to make a needle slimmer whenever insertion seems difficult. Slimming also reduces payload volume and resistance to buckling. These relationships can be illustrated without pretending to predict a real patch.

For an ideal, straight, circular column of unchanged material and length, Euler buckling load scales with the fourth power of radius. Reducing radius by 20% leaves about 41% of the original critical load, while leaving 64% of the original volume. Shortening the unsupported length by half gives four times the ideal critical load, provided the boundary conditions remain equivalent. Real microneedles taper, contain defects and hydrate; skin provides changing support. These calculations show competing tendencies, not a safe load or a clinical design rule.

The payload calculation has a second trap. In an ideal cone, the volume from the tip to a fraction x of the length is x³ of the total. The first half of the length contains only one eighth of the cone’s volume. The same relation holds for an ideal pyramid with geometrically similar cross-sections. If cargo is uniform, a photograph showing half the needle below a flat surface does not imply half the cargo is there.

Concentrating cargo near the tip can change that relationship. In a simple geometric illustration, putting all cargo uniformly within the tip-most half would place it all inside that region, rather than one eighth. But achieving the same total payload there would require eight times the local concentration. That may change stability, viscosity, strength or dissolution. Tip loading trades one constraint for another; it does not make the constraint disappear.

A study led by vander Straeten (2023; journal issue 2024) provides an experimental reason to take these distinctions seriously. With the tested polymer formulation, pyramid and conical needles both entered excised pig skin, but the reported volumes dissolved after ten minutes were 36% and 8%. The endpoint was disappearance of needle volume assessed from images. It was not a direct measurement of active vaccine recovered within tissue.

The analytical curve above cannot explain that difference on its own. Both ideal cones and pyramids follow the same cubic tip-volume relation. Actual tip angle, dimensions, insertion, contact and dissolution must be considered. The experiment is useful precisely because it separates a design that enters from one that releases more of its structure within a given time.

The reported difference depends on the fabricated geometry, material and conditions of application. It cannot be attributed to the name of the shape alone.

Follow the dose beyond the patch

Rouphael and colleagues’ human influenza trial (2017) enrolled 100 adults across four groups. Vaccine patches were worn for twenty minutes. Delivery was assessed using antigen remaining in used patches: estimated mean doses differed across the three influenza strains. This was a practical way to assess depletion during use. It was not a direct sample-by-sample measurement of antigen deposited in each participant’s skin.

06 · A measurement in people

What was left in the used patch?

Those measurements answer different questions. Material missing from a patch can be in the tissue, on the surface, in a dressing or outside the measured compartments. A tissue experiment can collect more of those compartments than a routine human vaccination study can. The opportunity is to account for where the material went and establish whether the recovered material retains the relevant activity.

Dose accounting distinguishes material remaining on the device, material outside the tissue and material recovered within it. An unexplained difference between loading and recovery does not identify where the missing material went.

A mass assay is also not a potency assay. Depending on the vaccine, molecular integrity, antigen recognition and cellular activity may require different tests. For mRNA, detecting RNA is distinct from demonstrating expression; for an antigen, detecting protein is distinct from preserving the properties needed for a response. An inert fluorescent tracer can help locate transport but cannot by itself establish vaccine performance.

What tissue experiments can establish

An excised tissue sample can show local entry, material release and response under the conditions of a particular experiment. It does not reproduce the full physiology of a living person or establish vaccine protection.

Pain needs its own evidence. Gill and colleagues (2008) measured pain in adult volunteers using stainless-steel microneedles; length mattered, while tip-angle changes did not show a consistent significant effect in the tested range. Ex vivo tissue cannot report pain. Force, strain, puncture geometry and local injury are useful measurements, but none should be relabelled “painless”. A human tolerability study would remain a separate step.

The design has to survive manufacture

A structure seen in nature must be made from a different material by a different process. Small features can change during filling, drying and removal from a mould. These constraints affect whether a biological mechanism can be reproduced consistently.

The vaccine-printer work separates cargo-bearing tip loading from a polymer-only backing and evaluates formulations for retained biological activity after drying. This is a useful precedent for treating process, material and geometry together. A geometry that works in one polymer cannot be assumed to work after changing the formulation to protect a different cargo.

Likewise, stability belongs to a formulation, package and storage condition. A result for one cargo or a short heat exposure does not establish the same performance for another product.

What the comparison shows

The mechanisms solve different problems. Alternating motion couples shape to movement. Barbs alter entry and retention. Swelling changes the structure after entry. Grooves provide a path for liquid. Each result belongs to a particular material, apparatus and tissue context.

For a dissolving vaccine patch, the relevant question is how those mechanisms relate to delivery of active material. Resemblance to a biological structure is a starting observation. Published evidence determines what can reasonably be inferred from it.