Part One of a Two-Part Series

Why Delivery is Half the Equation

In evaluating the efficacy of a cosmetic active, the late Dr. Albert Kligman—one of modern dermatology's foundational figures—identified three questions that must be answered before any performance claim can be credibly made:

Can the active ingredient penetrate the stratum corneum and reach the intended target layer of the skin at a sufficient concentration, within a timeframe consistent with its mechanism of action?

Does the active have a known, specific biochemical mechanism at the level of its target cells or tissue in human skin?

Is there published, peer-reviewed, double-blind, placebo-controlled, statistically significant clinical data to substantiate the claimed benefit?

Kligman's framework distills into a simple but demanding equation: clinical efficacy = active ingredient potency×delivery efficiency. An ingredient with exceptional in-vitro activity but poor skin penetration will underperform in practice. An ingredient with modest intrinsic potency but optimized delivery can outperform it.

This is why the cosmetic active delivery system has become, in the most rigorously developed products, as strategically important as the active itself.

What an Effective Delivery System Actually Does

Before exploring the mechanisms, it is worth being precise about what a delivery system is solving for. Seven distinct performance advantages accrue from a well-designed active delivery architecture:

Efficacy at the target site. Delivery ensures the active reaches its biological target—whether that is the stratum corneum, the viable epidermis, or the dermis—at a concentration sufficient to produce a measurable effect.

Penetration of barrier-resistant molecules. Many high-value actives have molecular characteristics—size, hydrophilicity, charge—that limit their ability to traverse the stratum corneum unaided. Delivery systems provide the vehicle.

Stability protection. Retinol, certain peptides, vitamin C derivatives, and other actives are prone to oxidative or photolytic degradation during manufacture, shelf life, or on-skin application. Encapsulation shields them from direct contact with destabilizing elements.

Controlled release. Sustained, gradual release from a carrier system extends the period over which an active is bioavailable at the target site, improving efficacy per unit of active and reducing the concentration peaks associated with irritation risk.

Tolerability management. This is particularly relevant for high-potency actives—retinoids, exfoliants, high-concentration antioxidants—where the delivery system can functionally decouple the formulation concentration from the skin-contact concentration.

Sensory and texture optimization. Carrier systems can enable the formulation of hydrophobic actives in lightweight aqueous vehicles, expanding the range of textures available for efficacy-focused formulations.

Differentiation. A proprietary delivery architecture is one of the few genuinely defensible sources of product differentiation in a category where active ingredient lists are frequently replicated.

The Target: Skin Anatomy Relevant to Delivery

Structural Overview

The epidermis is composed of two functional zones. The non-viable epidermis—the stratum corneum—is the principal barrier to percutaneous absorption. It consists of 5 to 15 layers of corneocytes embedded in a lipid matrix, with a total thickness of approximately 15 to 50 µm, low water content (5–20%), and metabolic inactivity. The lipid matrix—composed primarily of ceramides, cholesterol, and free fatty acids in a lamellar arrangement—provides the hydrophobic barrier that limits water loss from within and penetration from without. The stratum corneum is appropriately described as a "brick and mortar" structure: protein-dense corneocytes as bricks, lipid lamellae as mortar.

The viable epidermis (stratum granulosum, stratum spinosum, stratum basale) represents the second physical barrier—its tight junctions and adhesion proteins limit passage of molecules that have traversed the stratum corneum.

The dermis provides structural support through its collagen (approximately 70% of dry weight) and elastic fiber network, and contains the lymphatic and vascular systems through which dermally absorbed material is cleared or enters circulation.

The subcutaneous layer lies beneath the dermis, composed primarily of adipocytes, and functions as insulation and mechanical cushioning.

A Critical Regulatory Distinction: Dermal Delivery vs. Transdermal Absorption

This distinction is foundational for cosmetic ingredient development and must be understood clearly in regulatory terms.

Dermal delivery (the cosmetic scope) refers to the deposition and action of an active at the stratum corneum, viable epidermis, and/or dermis. The active exerts a local effect and does not enter systemic circulation in meaningful quantities.

Transdermal absorption (the pharmaceutical scope) refers to the penetration of a substance through the full skin barrier—including the dermis—into the systemic circulation, where it produces pharmacological effects elsewhere in the body.

In the European Union, UK, and US regulatory frameworks, a cosmetic product must not be designed to penetrate beyond the skin's surface layers in a way that produces systemic biological effects. Products or claims implying systemic action fall within the definition of a drug or medicinal product and are subject to pharmaceutical regulation. Cosmetic active delivery, in regulatory terms, operates entirely within the dermal delivery scope.

Three Pathways by Which Actives Enter the Skin

1. Intercellular (Lipid) Pathway

The primary route for most cosmetic actives. The intercellular lipid domains account for approximately 30% of stratum corneum volume, but—because their structural resistance is lower than that of the corneocytes themselves—they represent the dominant pathway for most penetrating molecules. Lipophilic and non-polar molecules traverse this route most readily, moving through the lipid lamellae via diffusion.

2. Transcellular Pathway

Direct passage through corneocytes, crossing both the lipid-protein envelope and the intracellular keratin matrix. This route is physically demanding and represents a minor fraction of total percutaneous absorption for most molecules. Water-soluble and polar actives face particular difficulty here, as both the lipid envelope and the intracellular architecture present hydrophobic barriers.

3. Appendageal Pathway

Hair follicles, sebaceous glands, and eccrine sweat glands provide an alternative route that bypasses the intact stratum corneum. This pathway is particularly relevant for larger hydrophilic molecules—including certain peptides and polysaccharides—that cannot traverse the intercellular lipid route efficiently. However, appendageal structures cover only approximately 1% of total skin surface area, limiting the quantitative contribution of this route under most conditions. It is, nonetheless, disproportionately significant for large-molecule actives and nanoparticulate delivery systems that accumulate in follicular openings.

Factors That Govern Skin Permeability

Understanding the variables that modulate dermal penetration is essential for formulation optimization. Seven key factors:

Molecular size and weight. The 500 Dalton Rule—derived from Lipinski's Rule of Five, originally developed at Pfizer to predict oral bioavailability—provides a useful heuristic: molecules above approximately 500 Da show a rapidly declining ability to penetrate intact skin via the intercellular route. Above approximately 1,000 Da, percutaneous penetration through intact healthy skin is generally considered negligible for practical formulation purposes.

Notable exception: research from the Medical Experiment Center at the China Academy of Chinese Medical Sciences demonstrated, using second harmonic generation imaging combined with two-photon fluorescence microscopy, that recombinant collagen with a molecular weight of approximately 58 kDa can be detected in follicular channels and, in part, in the dermis after 1 hour of topical application—traveling via the appendageal route rather than intercellular diffusion.

Lipophilicity. The principle of like-dissolves-like applies directly to skin penetration. Lipophilic molecules traverse the lipid-rich intercellular matrix more readily than hydrophilic ones. The contrast between water-soluble alpha-hydroxy acids (primarily surface-active) and lipid-soluble beta-hydroxy acids such as salicylic acid (follicular penetration, comedolytic activity) illustrates this in practice.

Active concentration. Percutaneous absorption follows Fick's First Law of Diffusion: flux is proportional to the concentration gradient across the membrane. Higher active concentration in the applied formulation drives faster penetration, all else being equal. This is the physiological basis for concentration-dependent efficacy claims, and the reason that delivery systems that improve apparent concentration at the skin surface—or maintain effective concentration over time—translate directly to improved outcomes.

Molecular polarity and charge. Polarity affects penetration independently of water solubility. Non-polar molecules penetrate more readily than polar ones, even where water solubility is comparable. Charged species face additional electrostatic barriers at the skin surface.

Skin temperature and hydration. Elevated skin temperature increases molecular kinetic energy and membrane fluidity, accelerating diffusion. Increased stratum corneum hydration—whether from occlusion, humectant application, or high-humidity environments—swells corneocytes, reduces the structural density of the lipid matrix, and enhances penetration of hydrophilic actives in particular. This is the mechanism underlying the enhanced efficacy of products applied after showering, or used under occlusive formats such as sheet masks.

Anatomical site. Stratum corneum thickness and appendageal density vary substantially across facial anatomy. Nasal folds demonstrate the highest absorption rates on the face; the forehead and chin at intermediate levels; the cheeks at the lowest. Thinner skin is more permeable than thicker skin—a consideration in formulations targeting delicate or periorbital areas.

Formulation vehicle and penetration enhancers. Because the skin barrier is so effective, formulation choices determine whether an active can reach its target in practice. Emulsified vehicles (oil-in-water or water-in-oil) provide inherent penetration-enhancing properties through the emulsifier fraction. Penetration enhancers—discussed in detail below—offer additional modulation of barrier resistance.

Five Principles of Percutaneous Penetration

1. Diffusion Theory

Passive diffusion, governed by Fick's First Law, is the primary physical mechanism driving most percutaneous absorption. Molecular thermal motion drives random movement; the net directional flux follows the concentration gradient. Practical approaches to increasing diffusion rate include: raising active concentration; using mild keratolytic agents to reduce stratum corneum thickness; applying surfactants to disrupt intercellular lipid organization; and facilitating follicular opening via sebum management.

2. Osmotic Pressure Theory

Skin functions as a semipermeable membrane. Osmotic pressure—the excess pressure required to maintain equilibrium across the membrane—can be expressed asπ= cRT (where c = solute concentration, R = gas constant, T = absolute temperature). Increasing active concentration or product temperature raises the osmotic driving force. Physical application methods—massage and occlusion—supplement diffusion-driven absorption by applying external pressure that exceeds the baseline osmotic pressure.

3. Hydration Theory

Hydrating the stratum corneum causes corneocyte swelling, which loosens the packed protein-lipid architecture and increases molecular permeability. Humectants (glycerin, butylene glycol, propanediol), occlusive formats, and application post-cleansing all leverage this mechanism. The effect is proportionally greater for hydrophilic actives than for lipophilic ones.

4. Like-Dissolves-Like Theory

Chemical similarity between an active molecule and the intercellular lipid matrix is a primary predictor of penetration efficiency. Oleic acid, isopropyl myristate, and similar fatty acid esters are incorporated into cosmetic formulations partly because they are structurally compatible with sebaceous lipids and stratum corneum lipid lamellae, facilitating co-penetration of accompanying actives. Lipid-soluble vitamins (tocopherol, retinol) penetrate more readily than water-soluble equivalents. Targeted delivery systems exploit this principle structurally: liposomes, with their phospholipid bilayer architecture, approximate the chemical character of cell membranes, enabling efficient fusion and active release at the target site.

5. Structural Modification Theory

Chemical penetration enhancers work partly by reversibly disrupting the ordered lamellar organization of stratum corneum lipids—increasing lipid fluidity and expanding the diffusion pathways available to penetrating molecules. The key criterion is reversibility: the stratum corneum should recover its normal barrier function after the enhancer is removed or metabolized.

Chemical Penetration Enhancers: Categories and Mechanisms

Chemical penetration enhancers increase percutaneous absorption by reversibly reducing stratum corneum barrier resistance. Their mechanisms cluster around four modes of action:

Disruption of the lamellar lipid structure, increasing lipid fluidity and the diffusion coefficient within the lipid domain

Extraction of stratum corneum lipid components, expanding intercellular channels

Interaction with keratin proteins within the corneocyte, modifying the transcellular pathway

Increasing the solubility of the active within the stratum corneum, raising the concentration gradient

The major categories of chemical penetration enhancers in cosmetic use:

Polyols (propylene glycol, ethanol, butylene glycol, pentylene glycol, inositol)—primarily function by increasing active solubility in the stratum corneum and by increasing skin hydration via hygroscopicity, thereby raising the osmotic driving force.

Fatty acids and esters (oleic acid, isopropyl myristate)—act via like-dissolves-like interaction with stratum corneum intercellular lipids, increasing fluidity and disrupting lamellar order. Their mechanism is structurally targeted to the lipid domains rather than the protein fraction.

Surfactants and solubilizers—dissolve intercellular lipids, expanding the channels through which actives diffuse, and reduce interfacial surface tension.

Azone (laurocapram) and sulfoxides—act by extracting extracellular lipids and reorganizing the lamellar architecture, substantially increasing flux. Azone is pharmacologically effective but carries a sensitization risk profile that limits its application in cosmetics designed for regular use on healthy skin.

Dimethyl Isosorbide (DMI)—a natural-derived solvent and penetration enhancer that fulfills the criteria of an ideal chemical penetration enhancer: pharmacologically inert; non-toxic, non-irritating, and non-sensitizing; reversible in action; compatible with a broad range of actives and co-solvents; and miscible with water in all proportions. DMI's mechanism centers on its capacity to dissolve sebum and stratum corneum surface lipids, enabling co-dissolved actives to reach effective concentrations within the viable epidermis. It is a particularly effective solvent and carrier for salicylic acid, resveratrol, idebenone, and resorcinol-based brightening actives.

Terpenes (phytol, menthol derivatives, bisabolol, essential oil components)—naturally occurring and clinically well-characterized penetration enhancers. The terpene class is notable for its efficacy at low concentrations (typically 1–5%) with minimal irritation, reversible interaction with stratum corneum lipids, and activity across both hydrophilic and lipophilic actives. Phytol, an acyclic diterpene alcohol, has been specifically studied as a transdermal penetration enhancer for both lipophilic and hydrophilic actives and presents an excellent tolerability profile relative to synthetic alternatives.

Nanohealth's Penetration Enhancer Platform

Nanohealth offers a focused range of penetration enhancer ingredients for cosmetic formulation, spanning the terpene, polyol, and solvent-enhancer categories:

Phytol—a terpene-class penetration enhancer derived from natural sources, supporting transdermal delivery of both lipophilic and hydrophilic actives with a demonstrated safety profile and minimal irritation potential at effective concentrations.

Inositol—a naturally occurring polyol-class penetration modulator, contributing both hydration-mediated barrier modulation and solubility-based concentration enhancement.

Dimethyl Isosorbide (DMI)—a natural-derived solvent-enhancer offering broad active compatibility, aqueous miscibility, and an established safety and tolerability profile in both cosmetic and pharmaceutical applications.

Coming in Part Two

This article has covered the structural biology of the skin as it relates to cosmetic active delivery, the three routes of percutaneous absorption, the factors that govern penetration efficiency, and the principles and categories of chemical penetration enhancement.

Part Two will move to Nanohealth's core technology area: carrier-based delivery systems—liposomes, nanoemulsions, supramolecular structures, and related formats—covering their architecture, the specific delivery problems each is designed to solve, and how they compare both to each other and to chemical enhancement approaches.