A new mini review published in Frontiers in Sensors by Sahil Malhotra of the UCD School of Mechanical and Materials Engineering and Arun H. S. Kumar of the UCD School of Veterinary Medicine in Dublin addresses this question head-on. Their analysis, which examines microneedle-based sampling of dermal interstitial fluid biomacromolecules, proposes a transport-aware and bias-conscious framework to guide the development of standardized and clinically robust microneedle-enabled interstitial fluid diagnostics. The review arrives at a critical moment, as microneedle technologies have achieved their greatest clinical success in continuous glucose monitoring but remain largely unproven for the larger biomacromolecules that constitute the foundation of most blood-based diagnostic assays.

The physiological constraints on accessing dermal interstitial fluid are more formidable than they might appear. The dermis behaves as a porous, hydrogel-like matrix where collagen and glycosaminoglycans bind water and create high hydraulic resistance while simultaneously imposing size- and charge-dependent hindrance that disproportionately affects proteins, antibodies, nucleic acids, and vesicles compared with small molecules. Even where interstitial fluid is abundant systemically, the accessible dermal volume per unit area is small and its replenishment is slow, constrained by high dermal fluidic resistance and lymphatic clearance. A substantial fraction of dermal fluid is not free but rather bound water associated with extracellular matrix components that is not readily extractable. This means bulk extraction approaches face a physiological speed limit: pulling fluid too quickly increases the risk that the collected material is no longer compositionally representative for larger biomolecules.

Partitioning from blood to interstitial fluid is not uniform across analytes either. Transport across continuous dermal capillaries is governed by endothelial junctions, glycocalyx charge effects, and vesicular transcytosis, so the interstitial fluid to blood ratio depends on size, charge, hydrophobicity, and protein-binding state. This is especially important for hormones and drugs that are highly protein-bound, where sensors may reflect only the free fraction, and for mid-to-large proteins ranging from 3 kilodaltons up to 1,000 kilodaltons that sit near size-selective thresholds. These transport physics impose fundamental limitations on bulk extraction strategies and complicate quantitative interpretation of any signal derived from sampled interstitial fluid.

Microneedle technologies offer a controlled and minimally invasive interface to the viable epidermis and superficial dermis by breaching the stratum corneum while largely avoiding vascular penetration. These micron-scale projections, typically ranging from about 250 to 1,000 micrometers in height, form transient microchannels into the viable epidermis or superficial dermis, supporting either in situ sensing or localized interstitial fluid extraction depending on device architecture. Needles shorter than about 300 micrometers primarily access the viable epidermis, whereas those in the 600 to 800 micrometer range more consistently penetrate into the superficial dermis, where interstitial fluid volume and vascular proximity are greater. However, increasing penetration depth does not necessarily translate to improved macromolecular recovery, as transport remains limited by extracellular matrix constraints, and deeper insertion increases the risk of nociceptor activation and potential blood mixture.

The review identifies six distinct microneedle-enabled transport mechanisms for accessing dermal interstitial fluid biomacromolecules, each with its own advantages and inherent biases. Passive bulk withdrawal through hollow needles hydraulically connects the interstitium to an external collector, with flow driven by local interstitial pressure, capillary forces in collection tubing, and transient fluid redistribution following puncture. One team assembled an array of five ultrafine nano pen needles in a 3D-printed holder and attached microcapillaries to each needle to collect small volumes of interstitial fluid from human forearm skin, yielding bulk samples suitable for downstream proteomics but limited to less than 20 microliters and necessitating high-sensitivity workflows. Capillary-driven extraction using porous polylactic acid microneedles fabricated from heat-bonded microspheres creates interconnected micropores that wick interstitial fluid by capillary effect, enabling rapid anti-SARS-CoV-2 IgG and IgM detection within minutes without external actuation, though transport through tortuous pores introduces size-dependent filtration and sensitivity to fluid viscosity.

Pressure-gradient-driven bulk extraction represents a more active strategy grounded in Darcy’s law-governed fluid flow in porous media. In one configuration termed Puncture-Out-and-Press, a solid microneedle array punctures the skin through aligned apertures in a collection plate, and after needle removal, external compression is applied to the surrounding skin while leaving the puncture sites uncompressed. This generates a lateral pressure gradient that directs interstitial fluid toward the punctures, where it accumulates and is collected via surface tension. Under optimized conditions, this approach enabled extraction of 15.5 plus or minus 7 milligrams of interstitial fluid within 5 minutes in human participants, corresponding to microliter-scale sampling volumes. The recovered fluid was subjected to untargeted LC-MS/MS proteomic profiling, identifying a broad interstitial fluid proteome and enabling quantitative comparison with plasma. The authors demonstrated that extraction efficiency and proteomic composition are sensitive to applied pressure distribution, with this method outperforming needle-in and vacuum-assisted variants in both yield and reproducibility.

Swelling hydrogel microneedles absorb interstitial fluid into a polymer network after insertion, with osmotic forces and polymer swelling drawing water and solutes into the expanding network. One team developed superabsorbent acrylate-based hydrogel microneedle patches that collect up to 6 microliters of dermal interstitial fluid in 10 minutes in vivo, enabling proteomic analysis from swelling-based collection. Importantly, the researchers explicitly identified that the extraction fingerprint is strongly dependent on microneedle chemistry, implying that hydrogel formulation does not merely collect interstitial fluid but selectively partitions biomolecules depending on polymer composition and interactions. Electro-assisted extraction combines reverse iontophoresis with hydrogel microneedle capture to enrich negatively charged cell-free DNA toward the collection interface, using reverse iontophoresis conditions of 10 volts for 10 minutes coupled with microneedle insertion. Affinity capture on functionalized microneedles represents a fundamentally different paradigm, relying on selective binding rather than bulk interstitial fluid extraction. One team developed a stellate silicon microneedle platform called ExoPatch, consisting of a 13 by 13 array of silicon microneedles each approximately 600 micrometers in height, coated with a thin alginate-polyvinyl alcohol hydrogel layer functionalized with Annexin V, which binds phosphatidylserine enriched on the membrane of tumor-derived exosomes. This system enabled sensitive detection of melanoma-associated exosomes in murine melanoma models and distinguished tumor-bearing from healthy samples using a lateral flow immunoassay readout.

Across these platforms, a consistent set of design trade-offs emerges between sampling efficiency, selectivity, and system complexity. Passive approaches, including hydrogel and porous microneedles, offer simplicity and minimal invasiveness but are constrained by diffusion-limited transport and low recovery efficiency for biomacromolecules. Active strategies such as pressure-assisted and electrokinetic systems improve extraction rates and access to larger or low-abundance analytes but require external actuation and may perturb the local interstitial fluid environment. Affinity-based platforms avoid bulk fluid extraction entirely by selectively capturing target molecules, enabling high specificity at the expense of representativeness. No single platform currently achieves high-efficiency, unbiased, and fully integrated sampling and sensing, highlighting the need for hybrid approaches that balance these competing design requirements.

The analytical challenge is not only whether biomacromolecules can be detected but what type of measurement each platform fundamentally produces and how that relates to physiological interstitial fluid composition. Volumetric sampling approaches are the most compatible with absolute or relative concentration measurements because they are based on physical fluid recovery, but this advantage depends on whether extraction volume is sufficiently defined and whether the recovered sample remains compositionally representative of native interstitial fluid. Capture-based systems do not primarily measure the composition of extracted interstitial fluid but rather the extent to which target molecules reach and bind to a functionalized interface, with output governed by local analyte flux, affinity interactions, binding capacity, and residence time rather than recovered fluid volume. Neither approach is intrinsically superior; bulk extraction is more appropriate when broader molecular profiling or downstream multianalyte analysis is desired, whereas capture-based systems are better suited to targeted detection when specificity is more important than representativeness. A key limitation across microneedle-based interstitial fluid biomacromolecule platforms remains the gap between analytical performance and clinical validation, as relatively few systems have been validated in humans using standardized protocols, and calibration between interstitial fluid and plasma remains a fundamental challenge due to size-dependent transport, protein binding, and local tissue effects. Future development should focus on resolving these gaps through coordinated advances in device design, analytical integration, and validation frameworks, with standardized sampling and reporting protocols that consistently document insertion depth, sampling duration, applied pressure, and transport mechanism to enable cross-study comparison and reproducibility.

Subject of Research: Microneedle-based sampling of dermal interstitial fluid biomacromolecules: transport constraints, design principles, and diagnostic translation

Article Title: Microneedle-based sampling of dermal interstitial fluid biomacromolecules: transport constraints, design principles, and diagnostic translation

Article References: Malhotra, S., & Kumar, A. H. S. (2026). Microneedle-based sampling of dermal interstitial fluid biomacromolecules: transport constraints, design principles, and diagnostic translation. Frontiers in Sensors, 7, Article 1819441. https://doi.org/10.3389/fsens.2026.1819441

Image Credits: AI Generated

DOI: 10.3389/fsens.2026.1819441

Keywords: microneedles, interstitial fluid, biomacromolecules, diagnostics, biosensors, dermal sampling, extracellular vesicles, proteomics, Microneedle-based, sampling, dermal, interstitial

The research was led by graduate student Koki Date, Assistant Professor Kosuke Kikuchi, and Professor Takafumi Ueno of the School of Life Science and Technology at Science Tokyo. Their focus was type 51 R-bodies, one of the best-studied varieties of these assemblies. Type 51 R-bodies are encoded by a compact four-gene operon containing the genes rebA, rebB, rebC, and rebD, which produce four proteins of the same names. Previous work had established that RebA and RebB are the principal structural components of the R-body architecture, forming the bulk of the ribbon itself. What remained unclear was the role of the two accessory proteins, RebC and RebD, and, more fundamentally, how all four proteins cooperate to produce the highly ordered architecture that extends from the nanoscale up to the microscale.

To dissect this process, the team combined genetic engineering with an extensive suite of biophysical and structural analysis techniques. They systematically constructed single-, double-, and triple-gene knockout mutants, deleting each of the four reb genes from plasmids one at a time and in various combinations, and expressed these mutant constructs in Escherichia coli. This approach allowed them to observe precisely what happens to R-body formation when any given component is missing. The resulting mutant assemblies were then examined in detail and compared against fully functional wild-type R-bodies, with the researchers assessing their morphology, internal structure, and protein composition using advanced microscopy, fluorescence assays, X-ray scattering, and spectroscopy.

The results overturned a simplistic view of R-body construction. It might have been reasonable to assume that RebA and RebB, as the dominant structural proteins, could self-organize into a functional R-body on their own. The experiments showed otherwise. When the assembly pathway was disrupted by the loss of either accessory protein, the system did not simply produce a smaller or weaker R-body; instead, it preferentially formed disordered aggregates rich in beta-sheet structure, the kind of misfolded clumping that renders a protein assembly useless as a mechanical device. In other words, the correct architecture is not an inevitable consequence of the main structural proteins being present. It depends on a regulated assembly pathway in which every one of the four proteins plays a distinct and necessary part.

RebD emerged from the study as a minor but indispensable building block. Although it makes up only a small fraction of the finished assembly, RebD guides the early stages of R-body formation, steering the growing structure toward an ordered conformation rich in alpha-helices. This early nucleation role appears to be critical: without RebD to set the assembly on the right track, the process veers into the disordered beta-sheet pathway, and no functional, extensible R-body results. The finding illustrates a principle familiar from many biological self-assembly systems, in which a scarce component acts as a scaffold or template that determines the fate of the much larger structure built around it.

RebC plays an even more surprising role, one that the structural analyses revealed by absence rather than presence. When the researchers examined the completed wild-type R-bodies, RebC was not detectable in the final structure at all. Yet when rebC was deleted from the system, the remaining proteins aggregated into shapeless clumps instead of organizing into the characteristic coiled ribbon. RebC, it seems, functions as a transient assembly factor: it is required for the other proteins to adopt the correct pathway during construction, but it does not remain as part of the finished machine. This kind of hit-and-run participation, in which a protein chaperones the formation of a structure without being incorporated into it, adds a layer of regulation that would have been invisible in studies focused only on the final product.

Taken together, the knockout experiments and structural comparisons demonstrate that the pH-responsive, extensible architecture of the type 51 R-body is a product of pathway regulation as much as of protein composition. The distinction matters because the mechanical performance of the R-body, its ability to remain coiled at neutral pH and to snap open explosively under acidic conditions, depends on the precise ordering of its components. An assembly that forms through the wrong route may contain the same proteins but lack the internal organization needed to store and release mechanical energy. As Professor Ueno explained, the experiments revealed that RebA and RebB alone are insufficient to produce a functional, extensible R-body, and that the characteristic pH-responsive architecture depends on a regulated assembly pathway involving all four proteins.

The broader significance of the work extends well beyond the biology of Paramecium symbionts. R-bodies belong to a growing class of stimuli-responsive protein materials that can sense changes in their chemical environment and convert them into mechanical motion. Such systems are attractive targets for biotechnology: protein-based actuators could in principle be used to deliver payloads across membranes, disrupt targeted cells, or build microscopic devices that respond autonomously to conditions such as the acidity of a tumor microenvironment or an infected compartment within a cell. Understanding how nature builds a working protein actuator from just four gene products provides a template for engineers attempting to design similar systems from scratch.

In that sense, the Science Tokyo team frames its findings as a set of design principles rather than merely a description of one bacterial curiosity. The study shows that a successful dynamic protein system requires attention not only to the final structure but also to the sequence of intermediate steps that lead to it, including the participation of components that guide assembly and then depart. As Ueno concluded, the work provides valuable design principles for engineering stimuli-responsive protein materials and microscopic protein-based machines capable of sensing environmental changes and generating mechanical motion. For a field that has spent seventy years wondering how R-bodies are made, the answer, it turns out, lies as much in the journey of assembly as in the destination.

The research, reported as an experimental study with the authors declaring no conflict of interest, marks a milestone in the effort to understand and ultimately harness one of biology’s most dramatic protein machines. By showing exactly why each of the four reb gene products is needed, whether as a dominant structural element, a helix-guiding nucleator, or a transient assembly factor that never appears in the final product, the study transforms the R-body from an enigmatic refractile curiosity into a tractable engineering platform. As synthetic biologists continue to seek molecular components that can sense, compute, and act at the nanoscale, the humble coiled ribbon of the Paramecium symbiont now stands as one of the best-understood examples of how evolution builds machines from protein alone.

Subject of Research: Assembly pathway of pH-responsive R-body protein assemblies in bacterial endosymbionts

Article Title: Uncovering the assembly mechanism of the extensible R-bodies

Article References: Uncovering the assembly mechanism of the extensible R-bodies. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: R-bodies, protein assembly, pH-responsive, Paramecium, RebA, RebB, RebC, RebD, protein engineering, biomacromolecules, self-assembly, synthetic biology

The material at the heart of the study comes from the sponge Haliclona sp., whose skeleton is built from spicules—slender, needle-shaped rods of biosilica. In earlier work, the same group showed that intact spicules, massaged briefly into the skin, create dense fields of nanometer-scale microchannels that boost penetration of small drugs. Those channels, however, proved too narrow for biomacromolecules, which typically range from a few to hundreds of kilodaltons. The researchers’ new idea was to etch the spicules themselves, dissolving away the tip and, unexpectedly, the entire axial core to leave a continuous hollow channel running the length of each needle. This transformation converts each spicule from a solid puncturing pin into something closer to a microscopic drinking straw embedded in the skin.

Manufacturing the hollow structures required careful optimization of an alkaline etching process. The team immersed purified spicules in sodium hydroxide solutions of varying concentration and duration, then evaluated how many spicules developed open tips, how large the resulting channels were, and whether the etched structures retained the mechanical strength needed to pierce skin. The harshest condition tested—2 molar sodium hydroxide for 72 hours—produced the widest openings and highest opening rates but visibly degraded structural integrity. The sweet spot turned out to be 2 molar NaOH for 48 hours, which maximized hollow-channel formation while preserving the spicules’ ability to withstand handling and skin insertion. Scanning electron microscopy of cross-sections taken at the tip, mid-shaft, and base revealed a continuous internal lumen, and confocal microscopy after Nile Red staining confirmed that the channels readily admit fluid and dissolved compounds.

The etching pattern itself has an elegant biological explanation. Siliceous sponge spicules are built around a proteinaceous axial filament, and the alkaline treatment appears to preferentially dissolve along this central core, carving out the hollow channel along a pre-existing biological template. This means the manufacturing process exploits the sponge’s own architecture rather than fighting it—a rare example of a natural biomaterial being upgraded through a one-step chemical modification into something functionally analogous to an engineered hollow microneedle array, but without any of the cleanroom microfabrication that synthetic microneedles demand.

With the hollow spicules in hand, the researchers benchmarked them against a formidable lineup of competitors in Franz diffusion cell experiments using fresh porcine skin: intact spicules, conventional microneedle patches, dermaroller devices, iontophoresis, and plain subcutaneous injection. For sodium fluorescein, a small hydrophilic tracer, hollow spicules alone achieved total skin absorption of roughly 24 percent, rising to nearly 43 percent when paired with a 15-minute course of low-current iontophoresis. The combination produced a synergistic index of 1.44, meaning the two techniques together outperformed the sum of their individual effects. For larger fluorescent dextrans of 20, 40, and 150 kilodaltons, the gap widened dramatically. With the 20-kilodalton dextran, hollow spicules plus iontophoresis pushed absorption to about 35 percent with a synergistic index of 3.29, while intact spicules combined with iontophoresis showed no synergy at all—clear evidence that the hollow channels, not merely the puncture sites, were doing the heavy lifting.

Dose dependence and distribution uniformity emerged as two further advantages. When the team varied the amount of hollow spicules applied, penetration of the 150-kilodalton dextran scaled accordingly, giving clinicians a tunable knob for dosing. More striking was the spatial pattern: biomacromolecules delivered through spicule-treated skin spread evenly across the entire treated area, with no significant concentration differences among randomly sampled sites. Subcutaneous injection, by contrast, produced a hot spot at the needle entry point with essentially no detectable drug 5 millimeters or 1 centimeter away. For depot drugs, that pooling is sometimes desirable, but for vaccines and immunotherapies meant to engage skin-resident immune cells, uniform distribution could translate into more consistent uptake and fewer local adverse effects.

The platform’s most clinically resonant demonstration involved an antibody fragment. The team compared hollow spicules against laboratory-fabricated dissolving microneedle patches—10-by-10 arrays of quadrangular pyramidal needles, each about 820 micrometers tall and mechanically robust enough to withstand roughly 0.8 newtons of compressive force—for delivering a shark-derived anti-PD-L1 variable new antigen receptor, a model antibody-like therapeutic. The hollow spicules achieved total skin absorption of about 25 percent, roughly doubling the microneedle patch result of 11.5 percent and more than quadrupling the untreated control. Because antibody drugs represent one of the fastest-growing classes of biologics yet remain almost entirely injection-bound, a topical route that outperforms microneedles would be a meaningful advance for patients who self-administer these therapies.

To test whether the effect matters biologically, not just chemically, the researchers delivered ovalbumin, a classic model antigen, to guinea pigs over consecutive days and tracked immune activation. Both the injection group and the hollow-spicule group showed visible immune reactions by day 2, and by day 8 the spicule group’s responses, while milder than injection, remained significantly stronger than controls and intact-spicule groups. Blood analysis told the same quantitative story: ovalbumin-specific IgE levels were markedly elevated in the spicule group, and interleukin-4 concentrations showed no significant difference from the injection group. In other words, a topical massage of sponge-derived microneedles provoked an immune response functionally comparable to a needle in the flesh—a result with obvious implications for needle-free vaccination.

Safety data were encouraging but appropriately caveated. Mild redness and swelling after application resolved within 48 to 72 hours, and histological counts of immune cells in treated skin spiked to roughly 550 cells per analyzed region at 24 and 48 hours before falling back to baseline levels by day 10, statistically indistinguishable from untreated controls. Repeated application produced no chronic inflammatory infiltration, and examination of major organs revealed no abnormalities. The authors are candid, however, about a longer-term concern: silica is a recognized granulomatogenic material, and cutaneous silica granulomas can appear months or even years after exposure. Their 10-day observation window cannot rule out such delayed reactions, and the team calls for extended follow-up studies, including polarized-light assessment for birefringent particles, before the platform moves toward human use.

The study also contributes a quantitative framework for predicting how drugs will behave under spicule treatment. Because hollow spicules remain embedded in skin during use, they generate annular channels rather than the funnel-shaped perforations left by removable microneedles. Building on established microporous-skin permeability models, the researchers derived and fitted an equation capturing the dependence of permeability on molecular weight and oil-water partitioning across six test compounds, with the expected inverse relationship to molecular weight dominating the tested range. The team notes that validation in ex vivo human skin and clinical studies will be essential, since animal skin differs from human skin in barrier structure and immune responsiveness. Still, the combination of natural abundance, batch-to-batch dimensional consistency, low manufacturing cost, tunable dosing, uniform drug spread, and demonstrated vaccine-grade immune activation makes hollow sponge spicules one of the more compelling entries yet in the race to retire the hypodermic needle.

Subject of Research: Transdermal delivery of biomacromolecules using hollowed marine sponge spicules as natural microneedles

Article Title: Enhanced skin delivery of biomacromolecules using hollow sponge spicules

Article References: Mou, D., Yang, M., Jia, M., Liu, Y., Chen, J., Xiao, X., & Chen, M. (2026). Enhanced skin delivery of biomacromolecules using hollow sponge spicules. Bioengineering & Translational Medicine, 11(5), Article e70149. https://doi.org/10.1002/btm2.70149

Image Credits: AI Generated

DOI: 10.1002/btm2.70149

Keywords: drug delivery, microneedles, sponge spicules, transdermal, biomacromolecules, vaccination, biologics, iontophoresis, biosilica, skin barrier, antibody delivery, marine biotechnology