Tag: particle size

  • The Powder That Raised a Sunscreen’s SPF Threefold Without Absorbing UV

    The Powder That Raised a Sunscreen’s SPF Threefold Without Absorbing UV

    On its own, the powder was useless as a sunscreen. A Thai team at Prince of Songkla University measured it plainly: formulations “containing 10% HAp alone, regardless of source, exhibited negligible UV protection (SPF = 1.03–1.13), failing to meet the Thai Industrial Standard requirement (TIS 2804–2017: SPF > 15).” If hydroxyapatite were being sold as a UV filter, that line would end the story.

    So what happened in the blend?

    The same failed powder was then combined with 5% Tinosorb® S, an organic filter that alone managed SPF around 11. Together: “the SPF values for SKJ- and TG-HAp hybrid systems reached approximately 29–30 (~180% increase relative to 5% TS). Notably, the SM-HAp system exhibited a higher SPF of 32.19 ± 1.46 (205% enhancement).” The mineral contributed almost nothing alone and tripled the result in combination — the study confirms “that HAp did not function as an intrinsic UV absorber.” The working mechanism is optical: particles scattering light through the film, stretching each photon’s path so the organic filter gets more chances to absorb it.

    Then the detail that matters for specification. Refining the winning powder — milling it “from irregular submicron aggregates (430 ± 140 nm) to a nanoscale distribution of 117.39 ± 61 nm” — lifted SPF “from 32.19 ± 1.46 to 35.15 ± 1.44,” and the accompanying UVAPF gain “was sufficient to elevate the protection grade from PA+++ to PA++++.” A label-grade improvement, bought entirely with particle-size control.

    There is a sourcing lesson in which powder won, too. Three fish-bone sources were compared and they did not perform identically as boosters — the spread between them was measured in whole SPF points. Feedstock, in other words, is a formulation variable. For a scattering application, everything that shapes particle geometry upstream — species, cleaning, calcination, milling — surfaces downstream as label grade, which is a more direct line from raw-material consistency to shelf claim than most ingredients ever show.

    What specification should a buyer pay for?

    Not “UV protection” — the mineral has none to sell. The value is as an SPF booster: a scattering aid that lets a formulator hit a target SPF with less of the expensive organic filter, or clear a marketing threshold the filter alone misses. And because the mechanism is optical, the specification that carries the value is physical, not chemical: particle-size distribution, aggregation state, dispersion behaviour in the film. Two lots with identical purity and different aggregate sizes are, for this application, different products — one is PA++++, one is not.

    The limits travel with the claim: every figure here is in vitro, measured on PMMA plates, and the authors state that for regulatory approval and commercial labelling, in vivo testing remains the global standard. The material was fish-bone-derived, from Thai seafood processing waste — another biogenic calcium stream finding a second market.

    The lesson generalises past sunscreen. When an ingredient’s job is physical, lot-to-lot consistency in the physical numbers is the whole purchase — and a certificate that reports only chemistry is describing a different product from the one you are buying.

    Sources

    Sookchoo P, Benjakul S, Prodpran T, Amnuaikit T, et al. “Synergistic Effects of Hydroxyapatite Derived from Fish Bone and Tinosorb® S on the UV Protection Performance of Sunscreen.” Cosmetics, 2026;13(2):71 (open access) — mdpi.com/2079-9284/13/2/71.

  • Two of the World’s Best Measurement Labs Sized the Same Particles — and Got Different Numbers

    Two of the World’s Best Measurement Labs Sized the Same Particles — and Got Different Numbers

    Two of the World’s Best Measurement Labs Sized the Same Particles — and Got Different Numbers

    In 2019, two national metrology institutes — Japan’s NMIJ and the United States’ NIST — ran what should have been the least surprising experiment in materials science. Both received particles from the same manufactured lot: polystyrene spheres, certified at 30, 50, 70 and 100 nanometres, about as well-behaved as particles get. Both used dynamic light scattering (DLS), a standard sizing technique. These are the laboratories other laboratories calibrate against. And as the study published in Metrologia reports, the apparent particle sizes were “invariably observed to disagree with each other.”

    Nothing Was Broken

    The uncomfortable finding was that both institutes were right. The instruments differed in laser wavelength, scattering angle, and sample concentration — and each of those quietly changes the number a DLS instrument reports. Only after the physicists corrected for every parameter did the two labs converge. Even then, the technique read the certified 29 nm particles as 33.2 nm — roughly 14% large — because DLS doesn’t measure the particle itself. It measures the particle plus the shell of water moving with it. The paper notes that international comparisons have generally found “a conspicuous difference” between DLS and methods that measure geometry directly, such as electron microscopy.

    The Population One Instrument Couldn’t See

    The stakes get clearer in a case documented by the EU and US nanomedicine characterisation laboratories. A sample measured by DLS showed a main peak at 87 nm — a tidy result. Laser diffraction on the same material found something else: a second population at 7.7 micrometres, nearly a hundred times larger, which DLS had missed entirely because large particles settle out of the laser’s path. One instrument said “uniform nano-suspension.” The other said “contains visible-scale aggregates.” Same sample.

    What the Gatekeepers Concluded

    Those two laboratories — the European Nanomedicine Characterisation Lab and the US National Cancer Institute’s characterisation lab, which screen materials before they reach regulators — drew a formal conclusion: “no single technique is capable to measure the particle size distribution” of every sample under every condition. Their standard practice is orthogonal measurement: multiple independent methods, cross-checked, with electron microscopy as the only direct witness to what crystals actually look like.

    What This Means at the Purchasing Desk

    Hydroxyapatite buyers live downstream of this physics. A “D50 of 5 µm” is not a property of the powder alone — it is a property of the powder plus the instrument, its settings, and the dispersion state of the sample. So the questions that matter on a datasheet are the ones metrologists ask each other: Which method produced this number? Dispersed or as-supplied? What are the D10 and D90, not just the median? Are there electron microscope images of this grade?

    The Number That Actually Predicts Performance

    If national laboratories need corrections to agree with each other, no buyer should compare two suppliers’ numbers at face value. What can be compared is consistency: whether a supplier’s distribution holds from lot to lot on the same declared method. That is a discipline of process control — the reason tightly controlled grades, such as Japanese eggshell-derived hydroxyapatite, are specified not for the figure on the sheet but because every delivery measures the same.

    Sources: Takahashi et al. 2019, Metrologia (open access via PMC); Caputo et al. 2019, Journal of Controlled Release (open access).