Review reports that iPSC-derived skin organoids self-assemble hair follicles and sebaceous glands, yet no study has used them for standardized Franz diffusion cell or IVPT permeation parameters

The review frames TDDS evaluation as the co-evolution of methodological standardization and skin-model renewal: Franz discussed the relevance of in vitro percutaneous absorption data to human skin in 1975; OECD TG 428 (2004) specified skin source, barrier integrity, receptor fluid, temperature control, dosing conditions and mass balance; USP <1724> brought in vitro performance testing of semisolid products into USP-NF; FDA first recommended IVPT in its 2014 product-specific guidance for acyclovir topical cream and issued a 2022 draft guidance on IVPT studies in ANDAs; and the EMA guideline adopted in 2024 and effective 2 April 2025 further refined donor number, replicate design, skin integrity, surface temperature, receptor medium and statistical acceptance criteria. It strings scattered regulatory and pharmacopeial milestones into a single narrative in which the limiting question shifts from whether permeation can be measured to whether the skin model faithfully represents the human barrier, and pairs this with Table 1 comparing RHE, full-thickness skin models, ex vivo human skin and SkOs across tissue architecture, appendages, dosing interface and polarity, barrier function, preparation time, reproducibility, cost and regulatory maturity. Based on public guidance and pharmacopeial documents from OECD, USP, FDA and EMA plus primary literature such as Franz 1975; this is literature synthesis within a narrative review, not a quantitative systematic review or meta-analysis.

The review reports that SkOs combine features rarely found together in traditional models: iPSCs induced with temporally controlled BMP4, TGF-beta inhibitors and FGF2 self-organize into epidermal-dermal layers with hair follicles and sebaceous glands; single-cell RNA sequencing shows major epidermal lineages including basal stem cells, spinous and granular cells plus dermal mesenchymal populations such as fibroblasts, adipocytes and glial cells; BODIPY staining supports lipid secretion by sebaceous glands; xenograft studies show organoid-derived hair follicles undergo autonomous anagen-catagen-telogen cycling in vivo; dermal cells express cranial neural crest markers PAX3 and SOX10; the transcriptomic profile after roughly 4-5 months in culture resembles second-trimester human fetal skin; and mature SkOs restrict permeation of water-soluble tracer dyes such as Lucifer Yellow while expressing FLG, LOR and IVL. It organizes SkO validation evidence across four scales, namely cellular atlas, appendage regeneration and cycling, molecular signatures and developmental-stage localization, and physical barrier and biochemical function, and notes that planar air-liquid interface culture as reported by De Henau et al. improves hair follicle vertical orientation and promotes barrier-related protein maturation and stratum corneum compaction, easing the topical-dosing limitation of early cystic SkOs. Drawn mainly from construction and characterization studies including Lee et al. 2020 in Nature, Lee et al. 2022 in Nature Protocols, Shafiee et al. 2024, De Henau et al. 2025 and Sun et al. 2025; this is structural-phenotype and molecular evidence rather than TDDS permeation-function validation.

The review explicitly reports an evidence gap: the authors' search identified no clear study using Lee-type hiPSC-derived SkOs as standardized skin barrier models in Franz diffusion cells or conventional IVPT with systematic measurement of cumulative permeation, steady-state flux, permeability coefficient or skin retention; it also identified no original study directly using SkOs for microneedle insertion, puncture-depth measurement, mechanical stability testing or post-enhancement release kinetics, and no study quantifying appendageal flux, follicular reservoir effects or follicle-mediated nanocarrier delivery using SkO appendages. It converts the common forward-looking claim that SkOs can serve transdermal evaluation into a checkable statement of evidence boundaries, and concludes that the current role of SkOs is disease modeling and a potential alternative platform rather than a mature pharmaceutical permeation model. Based on the authors' targeted search of PubMed, Web of Science, Scopus and Google Scholar covering publications to June 2026, described as a narrative rather than formal systematic review; as a negative search result its strength is bounded by databases and search terms.

The review groups translational bottlenecks into a systemic set: SkOs rely on stem-cell self-organization in three-dimensional culture with morphogenetic uncertainty producing batch-to-batch and within-batch heterogeneity, so parallel organoids in one batch can differ in size, epidermal thickness, hair follicle number, tissue polarity and maturity; Lee et al. report that differentiating human iPSCs into SkOs with mature hair follicle- and sebaceous gland-like structures typically requires more than 140 days of continuous culture whereas RHE models are often ready within 2-3 weeks; Matrigel is expensive and tumor-derived, creating batch-to-batch compositional variability; the FDA IVPT draft guidance gives a diffusion-cell example with an effective dosing area of approximately 1.77 cm2 while SkO effective exposure area is currently at the millimeter scale, with three-dimensional architecture, tissue polarity and surface continuity difficult to reconcile with conventional Franz diffusion cell or IVPT loading; and the HipSci project analyzing 711 iPSC lines from 301 healthy donors found that 5%-46% of variation in iPSC phenotypes could be attributed to inter-individual differences, while Shafiee et al. generated SkOs from three hiPSC lines that all formed complex organoids with stratified skin structures and pigmented hair follicles but differed in generation efficiency. It decomposes 'insufficient standardization' into actionable issues of production and quality control, economic cost and time efficiency, and biological maturity with iPSC line variability and genetic stability, and proposes that TDDS-oriented release criteria extend to epidermal thickness, stratum corneum maturity, tight-junction integrity, lipid and ceramide profile, appendage density, tissue polarity, effective diffusion area and baseline barrier-function readouts. The figures for time, area and variation proportions are cited from reviewed literature and guidance (Lee et al. 2020, the FDA IVPT draft guidance, Kilpinen et al. 2017, Shafiee et al. 2024), so they are literature citations rather than new measurements by this review.

The article is positioned as a narrative and forward-looking synthesis, suited to readers who want a rapid map of TDDS evaluation models, an understanding of the structural capability boundaries of SkOs, and preparation for model selection and quality-control indicator design, including those in transdermal formulation development, skin model and organ-on-a-chip construction, and research or submission work tracking non-animal method regulation. Its conclusions apply at the level of how to judge whether a model qualifies for standard IVPT, not at the level of permeation values for a given drug or formulation. The path it proposes is to first establish reproducible manufacturing and quality-control systems and TDDS-oriented release criteria, then advance permeation-parameter measurement, and compare SkO data against classical IVPT and ex vivo human skin data.

Readers should note that this is a narrative review based on a targeted search rather than a quantitative systematic review or meta-analysis, so 'not identified' statements are bounded by the databases, search-term combinations and the June 2026 cutoff. Quantitative permeation comparisons of SkO barrier maturity against adult ex vivo human skin, head-to-head comparisons among different SkO construction protocols, and direct comparisons of SkOs with skin-on-a-chip platforms for TDDS evaluation are all described as still lacking. The possibility that SkOs may overestimate drug permeation is explicitly framed in the text as an inference from barrier biology rather than a conclusion from standard TDDS experiments. Evidence for AI and computer vision in SkO quality control comes largely from intestinal, colonic or tumor organoids and skin imaging platforms, and whether image features can predict barrier-function indicators such as TEER, tracer permeation, drug flux, skin retention, lipid composition and tight-junction protein expression remains to be validated; PBPK, inverse mechanistic modeling, microneedle finite-element simulation and digital twins are technologies of differing maturity, and the text stresses that a single PBPK model or a static machine-learning predictor should not be described as a complete digital twin. In addition, the loaded text is an incomplete version: Figures 1 to 4 appear only as caption placeholders without image content, and the reference columns of Tables 1 and 2 contain gaps, so details tied to the figures and some citation attributions cannot be verified from the current text.