Microfluidics is a multidisciplinary and comprehensive technology capable of precisely controlling and manipulating fluids at the micrometer scale. Microfluidics has the advantages of low sample requirements, high throughput, low cost, and high sensitivity compared to traditional laboratory-based macroscopic techniques.
By combining microfluidics with cell biology, tissue engineering and biosensing technology, it has a promising application in the assessment of cosmetic safety and efficacy that deserves attention.
1. Physical and chemical testing applications
1) Heavy metal detection
Mercury is often added to cosmetics to achieve whitening effects, but overuse can cause chronic toxicity to the skin and organs. Biosensors have been created to address this problem. When Hg²⁺ is present, the probe is converted to G-quadruplex DNA and binds to hemoglobin to form the DNA enzyme.
The DNA enzyme reacts with the precipitated TMB to form a visible color band on the paper and the length of the band is positively correlated with the Hg²⁺ concentration. This sensor can be used not only for rapid visual screening of Hg²⁺, but also for the immediate detection of other metal ions.
2) Microbial detection
Microbial contamination in cosmetic products is a hot topic of concern for the society because contaminated microorganisms may produce metabolites that can lead to skin inflammation or allergic reactions. For this reason, a microfluidic system based on a blend of polydimethylsiloxane (PDMS), paper, and glass was developed in a related study.
The system utilizes a biosensor with integrated aptamer-functionalized graphene oxide for the direct detection of pathogenic microorganisms without the need for sample preparation procedures. This microfluidic biosensor device detects Staphylococcus aureus in less than 10 minutes, dramatically shortening the detection cycle, is more efficient compared to traditional culture methods, and shows great potential for rapid detection of other diverse bacterial and viral pathogens.
3) Antibiotic testing
Antibiotics commonly used in cosmetics include metronidazole, chloramphenicol, and ofloxacin, which inhibit skin microorganisms, enhance the skin's ability to resist bacterial infections, and provide surface protection.
However, prolonged use of antibiotic-containing cosmetics may lead to adverse reactions such as contact dermatitis, e.g., erythema, edema, oozing and burning sensation. In addition, it may trigger an increase in bacterial resistance and reduce the effectiveness of treatment.
Researchers have developed a microfluidic platform for electrochemical detection that enables simultaneous multiplexed analyses in clinically relevant samples, enabling simultaneous electrochemical readouts of different enzyme-linked assays for eight different analytes.
The device uses a highly sensitive biomolecular sensor system capable of detecting both tetracycline and streptozotocin, two commonly used antibiotics, in a sample within 15 minutes.
2. In vitro skin modeling
1) Skin Chip
Skin Chip simulates the three-dimensional microenvironment of real human skin by culturing skin tissues in a microfluidic system, controlling physical and biochemical parameters such as medium flow, mechanical force and biochemical concentration gradient, so as to construct functional three-dimensional skin tissues with skin hierarchical and accessory structures.
A typical skin microarray consists of a transwell scaffold or a porous membrane that is used to separate the skin model from the underlying culture medium, allowing macromolecules such as drugs or cytokines to diffuse into the skin model. Skin tissue is usually made from primary isolated cells, stem cell differentiated skin cells, or skin obtained from biopsy tissue.
The advantages of skin chips in the cosmetic industry are not only reflected in their ability to more realistically restore the structure of human skin, but also in their ability to detect specific functions such as skin morphology, activity, biochemical indicators, barrier function and permeability.
In addition, a variety of in-situ biosensors can be integrated on the chip to realize real-time skin function detection and pharmacokinetic response, providing richer functionality for cosmetic development and evaluation.
2) Security assessment
Toxicological assessment
Related reports indicate that mixed skin-neurological and skin-hepatic models can be combined with representative analytical methods for real-time quantitative skin sensitization analyses and potential hepatotoxicity assessments of dermally administered chemicals, thus helping to quantitatively assess the toxicological effects of chemicals on the skin.
Chemical Stimulation Assessment
Researchers have developed a high-fidelity epidermal microarray to directly culture and differentiate human keratin-forming cells within a microfluidic chip. The chip was primarily used to evaluate 10 chemicals known to be toxins and non-toxins through cytotoxicity, and to further evaluate stimulus responses such as the release of inflammatory cytokines.
3) Evaluation of efficacy
Active Ingredient Screening
During cosmetic development, a significant amount of time is spent screening effective active ingredients and their combinations. To improve efficiency, researchers have invented a microfluidic platform that is capable of growing large numbers of dermal fibroblast spheroids (DFS) in a bionic and dynamic culture environment.
In the proof-of-concept design, high-throughput screening of 12 different ingredients or combinations of ingredients can be realized on a single chip, which greatly improves the screening efficiency and shortens the cycle time of cosmetic development.
Evaluation of transdermal absorption
The ability of a cosmetic product to penetrate through the skin barrier is a crucial part of the development process and needs to be tested for transdermal absorption of its active ingredients. Compared to the traditional Franz cell diffusion method, the use of skin microarray testing not only avoids ethical issues and species differences, but also more accurately simulates the skin microcirculation and provides more valid data.
Anti-Aging Evaluation
The research team in question developed a whole-layer skin microarray based on a pump-free PDMS chip system for testing the anti-aging effects of Curcuma longa leaf extract (CLLE) as a cosmetic ingredient.
The results of the study showed that the barrier function of the skin model was significantly enhanced after treatment with 50µg/mL of CLLE, and analysis at the gene and protein level showed an increase in the expression of filamentous and internal batch proteins. This study suggests that this pump-less skin microarray model can be used in the cosmetic industry as an alternative to animal testing.
Whitening Test
After culturing normal human keratinocytes and normal human melanocytes in the chip, the resulting melanin epidermis was structurally stable with a multilayered structure and barrier function. The study also tested the irritation and permeation properties of four chemicals using this epidermal chip. The results suggest that this skin chip could be used as an alternative method for in vitro assessment of skin irritation or permeability as well as cosmetic assessment.
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