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In Vitro Analysis of Heavy Metal Adsorption by Zeolite Skin Care Formulations Using a Quality by Design Approach

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Overview

This Materials paper evaluates an experimental zeolite-containing cosmetic cream (3 g zeolite per 100 g cream) as an anti-pollution topical formulation, using a Quality by Design (QbD) workflow and the Franz diffusion cell model to quantify in vitro adsorption and retention of five heavy metals (Cd, Co, Cr, Pb, Ni) from a 0.04 ppm spiked acidic metal solution. A 2 × 2 × 2 full factorial design (membrane type Strat-M® vs silicone; dosage 10 vs 20 mg; dosage regimen infinite vs finite) followed by a 12-cell physiological-conditions permeation study (Strat-M® + PBS pH 7.4; 32 ± 1 °C; 12 h) was applied. The zeolite formulation adsorbed Cr, Co, and Cd statistically significantly more than the placebo (one-way ANOVA p = 0.026, p < 0.001, p = 0.004 in Experiment 1; F(11) = 26.88, 127.4, 35.59 with p < 0.001 for Cr, Co, Ni in Experiment 2), while Pb and Ni patterns were less consistent (Pb p = 0.147 in Exp 1; F(11) = 4.6, p = 0.58 in Exp 2; Cd F(11) = 11.03, p = 0.08 in Exp 2). The optimal QbD operating window (composite desirability D = 0.89, ≈ 0.8851 in the run summary) was Strat-M® membrane + finite dosage regimen + 20 mg dosage. The authors estimate ≈ 23% of the observed adsorption is specifically attributable to zeolite (the remainder reflecting cream-matrix retention). This is in vitro proof-of-concept mitigation evidence, not contamination occurrence data; the “metal concentrations” reported throughout are residuals from a deliberately spiked donor solution.

Key numbers

  • Sample basis (Section 2.1, p. 2; Section 2.2.1-2.2.2, p. 3): single prototype cream formulation, 3 g zeolite per 100 g cream, 17.3% lipophilic phase, herbal-derived ingredients including plant waxes and Rosmarinus officinalis leaf extract (Supplementary Table S1). Spiked donor solution: mixed aqueous Cd + Co + Cr + Pb + Ni at 0.04 ppm each. Receptor compartment in Experiment 1: diluted acidic metal solution mimicking interstitial-skin-fluid acidity. Receptor compartment in Experiment 2: PBS pH 7.4 (Na₂HPO₄ 5.98 g/L, KH₂PO₄ 0.19 g/L, NaCl 8.8 g/L, adjusted with H₃PO₄). Both experiments held at 32 ± 1 °C with continuous magnetic stirring. Endpoint timing differs by experiment: Experiment 1 (adsorption screening) measured residuals after 24 h of incubation (Section 3.3, p. 7); Experiment 2 (physiological permeation) ran 12 h (Section 2.2.2, p. 3; Section 3.4, p. 9).
  • Donor-spike concentration was 0.04 ppm per metal (Section 2.1, p. 2). For context, the printed residual donor + membrane concentrations in Table 4 (Experiment 2) are in the 0.3-2.4 ppm range, an order of magnitude above the donor-spike; the authors attribute the higher absolute levels in part to background metal content in the placebo cream’s herbal-derived ingredients (Discussion p. 10: “Pb levels detected in the placebo may be related to trace amounts naturally present in some herbal-derived raw materials … <10 ppm for Pb and Ni” per supplier specs).
  • Experimental factorial domain (Table 2, p. 6): Membrane (Silicone vs Strat-M®); Dosage (10 mg vs 20 mg); Dosage regimen (0 infinite vs 3 finite). NOTE: Table 2 prints the “Dosage” row values as 0 / 3 and the “Dosage regimen” row values as 10 / 20, which is the inverse of the body-text definitions (Section 3.1, p. 5: “dosage … 10 mg and 20 mg” and “dosage regimen … finite dosing conditions”). Table 3 column headers carry the same swap. The numerical values themselves are unaffected; only the column/row labels are transposed. The reading used throughout this page follows the body-text definitions.
  • Run-level residuals in the donor + membrane compartment (Table 3, p. 6; metal concentrations in ppm; CCD configuration with 12 runs, α = 2, 8 factorial points + 4 axial points). Columns rendered as (Run | Membrane | Dosage mg | Regimen | Cr | Co | Cd | Ni | Pb):
    • Run 1: Strat-M® | 10 | 3 | Cr 0.325 | Co 0.385 | Cd 0.415 | Ni 0.367 | Pb 0.385.
    • Run 2: Silicone | 20 | 3 | Cr 0.398 | Co 0.387 | Cd 0.399 | Ni 0.340 | Pb 0.358.
    • Run 3: Strat-M® | 20 | 3 | Cr 0.169 | Co 0.214 | Cd 0.227 | Ni 0.237 | Pb 0.289.
    • Run 4: Strat-M® | 10 | 0 | Cr 0.353 | Co 0.451 | Cd 0.441 | Ni 0.404 | Pb 0.396.
    • Run 5: Silicone | 10 | 3 | Cr 0.336 | Co 0.428 | Cd 0.431 | Ni 0.403 | Pb 0.404.
    • Run 6: Silicone | 10 | 0 | Cr 0.338 | Co 0.424 | Cd 0.436 | Ni 0.371 | Pb 0.393.
    • Run 7: Silicone | 20 | 0 | Cr 0.405 | Co 0.411 | Cd 0.397 | Ni 0.373 | Pb 0.313.
    • Run 8: Strat-M® | 20 | 0 | Cr 0.396 | Co 0.403 | Cd 0.406 | Ni 0.395 | Pb 0.345.
    • Run 9: Silicone | 10 | 3 | Cr 0.347 | Co 0.417 | Cd 0.425 | Ni 0.408 | Pb 0.412.
    • Run 10: Silicone | 20 | 3 | Cr 0.405 | Co 0.385 | Cd 0.389 | Ni 0.414 | Pb 0.397.
    • Run 11: Strat-M® | 20 | 3 | Cr 0.232 | Co 0.248 | Cd 0.285 | Ni 0.299 | Pb 0.302.
    • Run 12: Strat-M® | 10 | 3 | Cr 0.329 | Co 0.388 | Cd 0.402 | Ni 0.389 | Pb 0.393.
    • Lowest-permeation runs (greatest adsorption): Run 3 and Run 11 (Strat-M®, 20 mg, finite regimen), with Cr 0.169-0.232 ppm, Co 0.214-0.248 ppm, Cd 0.227-0.285 ppm (Section 3, p. 6).
  • Optimisation by desirability function (Section 3.2, p. 7): maximum overall D = 0.89 (composite 0.8851) at Strat-M® + finite regimen + 20 mg dosage. Individual analyte desirability values for Pb, Ni, Cd, Co, and Cr “consistently above 0.82 and reaching up to 0.95 for Pb.” The body text mentions a “30 mg” optimal in one sentence; this appears to be a typographic inconsistency with the 20 mg upper bound of the factorial domain and is documented in Verification notes.
  • Experiment 2 (Section 3.4, p. 9; physiological permeation; 12 Franz cells with Strat-M®; receptor PBS pH 7.4; no receptor sampling — donor + membrane residuals only). Donor + membrane residual concentrations after 12 h (Table 4, p. 9; ppm):
    • Cell 1A (placebo + memb): Pb 2.423, Cd 0.561, Co 0.589, Cr 0.810, Ni 0.515.
    • Cell 2A (zeolite + memb): Pb 1.270, Cd 0.35, Co 0.377, Cr 0.582, Ni 0.405.
    • Cell 3A (placebo + memb): Pb 1.308, Cd 0.816, Co 0.569, Cr 0.653, Ni 0.546.
    • Cell 4A (zeolite + memb): Pb 1.137, Cd 0.465, Co 0.416, Cr 0.526, Ni 0.454.
    • Cell 5A (placebo + memb): Pb 1.830, Cd 0.517, Co 0.523, Cr 0.666, Ni 0.548.
    • Cell 6A (zeolite + memb): Pb 1.039, Cd 0.477, Co 0.381, Cr 0.541, Ni 0.473.
    • Cell 1B (zeolite + memb): Pb 0.906, Cd 0.462, Co 0.329, Cr 0.498, Ni 0.453.
    • Cell 2B (placebo + memb): Pb 1.078, Cd 0.466, Co 0.570, Cr 0.626, Ni 0.535.
    • Cell 3B (zeolite + memb): Pb 0.908, Cd 0.333, Co 0.384, Cr 0.534, Ni 0.497.
    • Cell 4B (placebo + memb): Pb 1.242, Cd 0.605, Co 0.547, Cr 0.777, Ni 0.576.
    • Cell 5B (zeolite + memb): Pb 0.896, Cd 0.375, Co 0.411, Cr 0.532, Ni 0.441.
    • Cell 6B (placebo + memb): Pb 1.374, Cd 0.613, Co 0.564, Cr 0.812, Ni 0.579.
  • Experiment 2 ANOVA (Section 3.4, p. 9): Cr F(11) = 26.88, p < 0.001 (significant); Co F(11) = 127.4, p < 0.001 (significant); Ni F(11) = 35.59, p < 0.001 (significant); Cd F(11) = 11.03, p = 0.08 (trend, not significant at α = 0.05); Pb F(11) = 4.6, p = 0.58 (not significant).
  • Experiment 1 statistics (Section 3.3.2, p. 8): one-way ANOVA between zeolite-treated and placebo formulations: Cr p = 0.026, Co p < 0.001, Cd p = 0.004 (all significant); Pb p = 0.147 and Ni p = 0.58 (neither significant). Coefficients of determination (R²) exceeded 0.9 for Cr, Co, and Cd. Tukey HSD post-hoc confirmed consistency across replicates.
  • Adsorption attribution (Section 4 Discussion, p. 10): “approximately 23% of the observed adsorption is specifically attributable to the presence of zeolite” relative to placebo. The remainder reflects matrix retention by the cream base itself.
  • Background metal levels in the placebo cream from herbal-derived ingredients (Section 4 Discussion, p. 10): supplier specifications report < 10 ppm for both Pb and Ni in the herbal raw materials used. These background levels affected absolute residuals but not the relative zeolite-vs-placebo comparison.
  • Selectivity ranking (Section 4 Discussion, p. 10): zeolite preferentially binds smaller-hydrated-radius, higher-charge-density cations: Cr³⁺ > Co²⁺ > Cd²⁺ ≫ Ni²⁺ ≈ Pb²⁺. Pb’s weaker binding attributed to lower hydration energy and formation of weakly soluble Pb complexes at the acidic pH used; Ni’s weaker binding attributed to competing affinity for organic functional groups in the cream base.

Methods (brief)

A prototype zeolite-containing cream (3% zeolite w/w, 17.3% lipophilic phase, herbal-derived plant-wax and Rosmarinus officinalis leaf-extract components per Supplementary Table S1) and its matched placebo were compared in vitro using vertical Franz diffusion cells. Donor compartment held cream + spiked acidic metal solution (Cd + Co + Cr + Pb + Ni at 0.04 ppm each). Two synthetic membranes were tested — Strat-M® and medical-grade silicone, 15 cm × 20 cm × 0.025 cm (Bioplexus, Boston, PA, USA; Strat-M® supplied by Merck Millipore, Burlington, MA, USA). Reagents: PBS buffer salts (Na₂HPO₄, KH₂PO₄, NaCl, H₃PO₄) from Supelco (Bellefonte, PA, USA). Experiment 1 (adsorption screening) used a diluted acidic metal solution in the receptor compartment to mimic interstitial-skin-fluid acidity for preliminary zeolite-vs-placebo screening; residuals were measured after 24 h of incubation (Section 3.3, p. 7). Experiment 2 (physiological permeation) used PBS pH 7.4 in the receptor compartment, 12 Franz cells (6 zeolite, 6 placebo, all Strat-M®), 20 mg dose, 0.4 mL acidic metal solution applied to the donor, 12 h duration (Section 2.2.2, p. 3; Section 3.4, p. 9), 32 ± 1 °C, continuous magnetic stirring; no receptor sampling — the analysis focused on residuals in the donor + membrane compartments. Quantification by inductively coupled plasma-optical emission spectroscopy (ICP-OES) after ultra-pure nitric-acid digestion (2-5% v/v), 0.22 µm filtration, and dilution-corrected calibration against certified multi-element standards. Statistical analysis: one-way ANOVA with Tukey HSD post-hoc; coefficient of determination R² for response-surface model fit; desirability function for global optimization. Software: MINITAB® 19.2020.1 (Section 2.4, p. 3-4). aQbD workflow: Analytical Target Profile (ATP), Critical Analytical Attributes (CAA), Critical Method Variables (CMV), Ishikawa cause-and-effect mapping (Figure 1, p. 4), 2 × 2 × 2 full factorial DoE, central composite design (12 runs), response-surface analysis, and desirability-function optimization. Stated limitations: in vitro only (Franz cells, synthetic membranes — not human skin); no receptor sampling in Experiment 2 (cannot quantify what crossed the membrane, only what was retained donor-side); single prototype formulation (n = 1 product, not a sample frame of marketed creams); 0.04 ppm donor spike is a single-concentration screen, not a dose-response curve; only five metals tested; no inorganic-vs-organic speciation of Cr (Cr-VI not measured separately from total Cr); 12 h endpoint only (no time-course beyond that); no in vivo validation; placebo cream itself contained trace metal content from herbal-derived raw materials (< 10 ppm Pb and Ni per supplier specifications), inflating absolute residuals.

Implications

  • Certification (HMTc): This paper is in vitro mitigation / efficacy evidence for an active cosmetic ingredient (zeolite) tested in a prototype anti-pollution skin care cream, not contamination occurrence on a marketed product. It does not contribute to occurrence pools for any HMTc product-category row. Its relevance to HMTc is on the mitigation side: it offers mechanistic and statistical support for zeolite (and similar negatively-charged aluminosilicate adsorbents) as a candidate active for anti-pollution leave-on cosmetic formulations, with selectivity strongest for Cr, Co, and Cd and weakest for Pb and Ni. If a future HMTc cosmetics ratchet program incentivises anti-pollution actives via lower thresholds or premium tiers, the data here are usable as one of several efficacy anchors. The “23% of adsorption attributable to zeolite” figure also flags that herbal-derived cosmetic matrices carry meaningful intrinsic metal background even before any active is added — a relevant input for raw-material sourcing standards on cosmetic ingredient lines.
  • Courses: Useful teaching case for (a) Franz diffusion cell methodology and the choice between Strat-M® and silicone synthetic membranes, (b) the QbD / aQbD framework (ATP → CAA → CMV → DoE → response surface → desirability) applied to a topical-product efficacy question, (c) the distinction between mitigation-efficacy studies and contamination-occurrence studies, (d) the bias risk introduced when a placebo contains intrinsic metal background from herbal raw materials, and (e) Hofmeister/hydration-energy reasoning for why zeolites preferentially bind Cr³⁺ and Co²⁺ over Pb²⁺ and Ni²⁺.
  • App: Out of scope. This is in vitro adsorption-method data on a prototype, not a measured contamination level on a consumer product. The app does not consume Franz-cell residuals.
  • Microbiome (if applicable): Not directly addressed. Zeolite’s effect on skin microbiome is mentioned only by implication (the formulation forms a “continuous film” and “transient metal scavenger” layer, Section 2.1, p. 3; Section 4, p. 10) but no microbiome endpoints are measured.

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