A revolutionary approach to assessing skin penetration that simultaneously measures compound delivery and biological effectiveness
Imagine applying your favorite anti-aging cream tonight. You can feel its silky texture and perhaps even imagine its active ingredients working their way deep into your skin. But how can scientists actually know whether those precious compounds are penetrating effectively to where they're needed?
For decades, this has been one of the most challenging puzzles in skincare and pharmaceutical development. The skin is designed to be a remarkable barrier—our body's first line of defense against environmental threats 2 . While essential for protection, this very quality makes it difficult to deliver beneficial compounds effectively.
Traditional testing methods have struggled to accurately mimic human skin while providing reliable penetration data. But now, a revolutionary approach is changing the game. Researchers at Soongsil University in Korea have developed an innovative 3D Agarose-Well system that promises to transform how we evaluate skin penetration 1 7 .
This cutting-edge technology not only provides more accurate measurements of how compounds travel through skin but simultaneously confirms their biological effectiveness—all within a lab-grown skin model that closely resembles human tissue.
To appreciate this breakthrough, we first need to understand why measuring skin penetration has been so challenging.
Our skin consists of multiple layers, with the outermost stratum corneum acting as the primary gatekeeper. This thin layer is composed of corneocytes (skin cells) surrounded by lipid bilayers, creating what scientists describe as a "brick and mortar" structure 2 .
Compounds can potentially travel through three pathways:
For years, researchers have relied on methods like Franz diffusion cells and trans-well systems to study skin penetration 2 6 . These approaches typically use either excised animal/human skin or reconstructed skin models. While valuable, they come with significant limitations:
The collagen-rich dermal layer often shrinks during testing, altering the skin's natural structure and barrier properties 1 .
Most methods can't evaluate how penetrated compounds actually affect living skin cells 1 .
Conventional systems struggle to maintain the structural integrity of skin equivalents throughout experiments 5 .
The research team at Soongsil University tackled these challenges by reimagining the very container in which skin models are grown. Instead of traditional plastic wells, they developed a customizable 3D Agarose-Well system that integrates directly with reconstructed human skin equivalents 1 .
At its core, the technology uses agarose—a natural polysaccharide derived from seaweed—that has been modified with octadecyl (C18) alkyl chains 5 .
This modification might sound like chemical jargon, but its purpose is revolutionary: by attaching these carbon-rich chains to the agarose molecules, scientists can fine-tune the physical properties of the gel to better mimic human skin.
The modified agarose structure entangles collagen fibrils within its matrix, preventing the contraction that plagues conventional methods 1 .
By adjusting the ratio of regular agarose to alkyl-modified agarose, researchers can control the well's hydrophilic/hydrophobic balance and mechanical strength 5 .
This entanglement creates a robust skin equivalent that maintains its architecture throughout the entire testing period 1 .
But what truly sets this system apart is its dual capability—it simultaneously measures how much of a compound penetrates the skin and assesses its biological effects on living skin cells 7 . This combination has been exceptionally difficult to achieve with previous methods.
To validate their system, the research team designed a comprehensive experiment comparing the penetration and effects of different compounds 1 .
The process began with creating a complete skin equivalent within the 3D Agarose-Well:
A collagen solution containing human dermal fibroblasts was introduced into the Agarose-Well, where it diffused uniformly throughout the matrix.
Keratinocytes (the primary cells of the epidermis) were seeded onto the dermal layer.
The developing skin equivalent was exposed to an air-liquid interface, triggering the keratinocytes to differentiate into multiple layers, including a protective corneous (outer) layer—similar to how actual skin matures 1 .
The researchers used transepithelial electrical resistance (TEER) measurements to monitor the development of the skin barrier. This technique measures electrical resistance across the tissue, indicating how well-formed the protective layers have become.
The TEER values increased from about 18 Ω·cm² for the dermis alone to approximately 100 Ω·cm² after 13 days of air-liquid interface culture, demonstrating the formation of a competent skin barrier with well-developed tight junctions 1 .
With the mature skin equivalent ready, the team applied four different compounds to its surface:
A known anti-wrinkle agent with a relatively low molecular weight
Containing a skin-penetrating peptide (SPP) sequence
Also containing an SPP sequence but structurally different from Peptide 1
Similar in size to Peptides 1 and 2 but lacking SPP sequences 1
The researchers then measured how much of each compound successfully penetrated through the full thickness of the skin equivalent over time.
| Compound | Molecular Characteristics | Final Penetration (%) | Time to Full Penetration |
|---|---|---|---|
| Adenosine | Low molecular weight | ~100% | ~24 hours |
| Peptide 1 | Contains SPP sequence | ~100% | ~24 hours |
| Peptide 2 | Contains SPP sequence | ~100% | ~24 hours |
| Peptide 3 | No SPP sequence | Significantly lower | >24 hours (incomplete) |
| Compound | Kinetic Rate Constant (k) | Diffusional Exponent (n) |
|---|---|---|
| Adenosine | ~34 | Not reported |
| Peptide 1 | 33.4 | 0.49 |
| Peptide 2 | 9.6 | Not reported |
| Peptide 3 | 0.11 | Not reported |
The results revealed a fascinating insight: despite Peptide 1 having a molecular weight approximately four times greater than adenosine, both compounds showed remarkably similar penetration rate constants (about 39 for Peptide 1 vs. 34 for adenosine) 1 7 .
This demonstrates that molecular weight isn't the only factor determining skin penetration—the presence of specific skin-penetrating peptide sequences can dramatically enhance a compound's ability to traverse the skin barrier.
Beyond mere penetration, the research team went further to answer a crucial question: Do the compounds that penetrate actually do anything beneficial to the skin?
They measured the production of procollagen—a precursor to collagen that indicates enhanced skin health and anti-aging effects. The results were clear: adenosine significantly boosted procollagen synthesis by approximately 23% compared to the control group 1 7 .
This finding is significant because it demonstrates that the 3D Agarose-Well system can simultaneously confirm both the delivery of an active ingredient and its biological effect—addressing two critical questions in a single experiment.
| Compound Tested | Procollagen Synthesis |
|---|---|
| Adenosine | ↑ ~23% vs. control |
| Control (untreated) | Baseline level |
This advancement represents more than just an incremental improvement in testing methodology. The 3D Agarose-Well system has far-reaching implications across multiple fields:
The ability to simultaneously verify penetration and efficacy could accelerate development of new skincare ingredients while reducing reliance on animal testing. Companies can now make more informed decisions about which compounds warrant further development, potentially bringing more effective products to market faster 7 .
Beyond cosmetics, this technology holds promise for transdermal drug delivery—medications administered through skin patches. The system could help researchers design better formulations that optimize drug penetration, potentially benefiting conditions ranging from hormonal disorders to chronic pain 2 .
The platform's ability to maintain stable, functional tissue models suggests applications far beyond skin penetration studies. Similar approaches could be adapted to model other tissues, potentially advancing disease modeling and regenerative therapies 1 5 .
| Material/Reagent | Function | Significance |
|---|---|---|
| Agarose-g-C18 | Hydrophobically modified agarose for well construction | Enables tunable physical properties to match skin characteristics |
| Human dermal fibroblasts | Primary cells for dermal layer | Creates living, biologically active dermal tissue |
| Human keratinocytes | Primary cells for epidermal layer | Forms stratified, barrier-competent epidermis |
| Type I collagen | Extracellular matrix component | Provides structural scaffold for skin equivalent |
| Skin-penetrating peptides (SPPs) | Penetration enhancers | Facilitate transport of compounds across skin barrier |
| Adenosine | Reference active compound | Benchmark for assessing anti-aging effects |
The development of the tailored 3D Agarose-Well system represents a significant leap forward in how we study and understand skin penetration.
As this platform continues to evolve, it may help unlock new possibilities in personalized skincare, where formulations could be tailored to individual penetration profiles. It might also contribute to reducing animal testing in cosmetic development by providing more human-relevant alternatives 8 .
The next time you apply your skincare serum, remember that there's fascinating science working to ensure its ingredients can effectively navigate the incredible barrier that is our skin. Thanks to innovations like the 3D Agarose-Well system, we're developing a deeper understanding of that journey—one that promises to yield more effective and scientifically grounded products for years to come.