Lab-Grown Skin Enables Real-Time Measurement of Cellular Responses to Test Substances

Lab-Grown 'Reporter Skin' Could End Animal Testing for Cosmetics Forever

The EU has banned animal testing for cosmetics and prioritizes non-animal alternative methods for risk assessment of new chemical substances. Researchers at the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB have now developed a groundbreaking three-dimensional skin model that directly displays the skin's reaction to substances: the "reporter skin." Thanks to its built-in reporter system, cellular responses can be measured with precision and speed—all in a living model. This innovation allows for effective testing not only of cosmetics but also of allergens, inflammatory agents, and toxic effects of biocides, pesticides, and chemicals.

Shampoos, creams, and makeup contain a wide range of substances. Before a new cosmetic product reaches the market, its ingredients must be tested for safety and efficacy, as required by European legislation. Another EU-wide regulation strictly prohibits animal testing for this purpose, and since 2013, cosmetics tested on animals have been banned from sale in the EU. Under the European Chemicals Regulation (REACH), plant protection products, biocides, and all other chemicals must also be assessed for potential hazards—such as allergenic or inflammatory effects—preferably without animal testing.

Non-animal alternative methods are therefore in high demand. Manufacturers often use lab-cultured skin cell lines when developing and testing new ingredients or formulations. In these cultures, skin cells grow in a petri dish with a nutrient-rich medium that surrounds them completely, making it difficult to test solid or oily substances. While lab-grown three-dimensional skin models are already commercially available, they have a key limitation: to analyze a test substance's effects, the tissue must be prepared and examined under a microscope. For investigating skin irritation that may only appear after repeated exposure, an enormous number of identical skin models is required.

Enhanced Predictive Power with a 3D Skin Model Featuring an Intact Skin Barrier

Researchers at the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB have established a three-dimensional human skin model that closely replicates the complex physiology of natural skin, including an intact barrier function. This patented model yields far more meaningful results than single-layer cell cultures. It consists of dermal fibroblasts—cells from the deeper skin layer embedded in a collagen matrix—as well as keratinocytes, which form a fully differentiated, multi-layered epidermis. "Like natural skin, this model develops a stratum corneum that acts as an effective barrier against external influences," explains Dr. Anke Burger-Kentischer, head of the Cell and Tissue Technologies department at Fraunhofer IGB.

The cells used in the skin model—fibroblasts and keratinocytes—are isolated from skin samples obtained as surgical byproducts in medical practices and hospitals. Unlike commercial cell lines derived from tumor tissue, these primary cells, directly sourced from native tissue, closely resemble in vivo cells and exhibit normal physiological behavior. The isolated cells are first immortalized to ensure they can be cultured indefinitely, providing a consistent and donor-independent supply for reproducible skin model production.

Precise, Real-Time Testing on a Living Model Through Reporter Functionality

The Key Innovation: A Built-In Reporter Function

The real breakthrough of this skin model lies in its integrated reporter system. Using so-called reporter genes, the expression of a target gene can be monitored both simply and reliably. "We link the reporter gene to the signaling pathway of a cellular cascade that plays a role in skin stress responses, then stably anchor the construct in the genome of our immortalized keratinocytes," explains Anke Burger-Kentischer, describing her groundbreaking approach. In inflammatory reactions, for example, the receptor—part of the signaling cascade—acts as the cell membrane's interface with the outside world: when the test substance binds to the receptor, it triggers the cascade, and through the coupling of the reporter gene to a transcription factor (the final domino in the cellular response), the reporter gene is transcribed and the reporter protein produced.

Burger-Kentischer employs secreted alkaline phosphatase as one of her reporters—an enzyme that converts its substrate into a yellow dye. "For our reporter skin, this means: after applying the test substance, we take a sample from the culture supernatant of the skin model, add the alkaline phosphatase substrate, and can measure the color change within minutes to hours if the signaling cascade has been activated," she explains. Since the cells in the reporter skin do not need to be destroyed for analysis, the temporal progression of the cellular response can also be tracked.

"Our Reporter Skin Mimics In Vivo Conditions and Enables Real-Time Analysis"

"With our reporter skin, we can realistically replicate in vivo conditions and analyze cellular responses in real time," says Burger-Kentischer. "This makes it a highly specific and cost-effective alternative to existing models, as the tissue does not require multiple labor-intensive steps—fixation, immunohistochemical staining, sectioning, and microscopy—to detect and evaluate cellular changes."

Suitable for Water-Repellent and Solid Substances

"Our reporter skin yields far more meaningful results than models lacking a skin barrier," she adds. "Unlike submerged cell cultures, it allows us to test a much broader range of chemicals and mixtures—including hydrophobic substances that do not form stable dispersions in aqueous culture media." For cosmetics and chemical manufacturers, this means that the 3D reporter skin model can, for the first time, be used to study oils, oil mixtures, and even solid materials such as textiles and food products.

Detecting Cytotoxicity, Sensitization, Inflammation, and Skin Penetration in a Single Model

Burger-Kentischer and her team have even succeeded in integrating multiple cellular signaling pathways—each with different reporters—into the same skin model. A full suite of 3D reporter skin models for various applications is now available.

"This allows us to assess a substance's toxicological potential, investigate its skin penetration, and specifically and rapidly detect the activation of different cellular stress pathways—all within the same model," she explains. In terms of applicability and predictive power, these models far surpass existing test systems on the market.

By mapping all three known skin stress signaling pathways, cosmetics and chemical manufacturers can quickly determine whether—and what type of—cellular stress a substance induces: whether it triggers inflammation, sensitizes the skin (potentially leading to long-term allergies), or induces endoplasmic reticulum (ER) stress, which causes proteins to misfold and lose their biological function.

Further Functional Assays Can Be Integrated

Before producing the in vitro models in large quantities for interested companies, Burger-Kentischer's team can first verify their suitability in the institute's own laboratory. Upon request, the reporter test system can also be expanded to include additional cellular signaling pathways or adapted for other organs.