Cytotoxicity Testing of Medical Dressings: Standards, Methods, and Regulatory Compliance

Cytotoxicity testing is a core biocompatibility requirement for medical dressings that contact breached skin, wounds, or compromised surfaces, verifying that leachable substances do not harm living cells. Performed in accordance with ISO 10993-5 and ISO 10993-12 (and their regional equivalents such as GB/T 16886.5), these in vitro assays are a mandatory element of biological evaluation for product registration worldwide. This article explains the technical principles, sample preparation, test methods, acceptance criteria, and typical applications of cytotoxicity testing for medical dressings.

Technical Overview

Cytotoxicity testing evaluates whether chemical substances released from a medical dressing, such as residual monomers, plasticizers, processing aids, sterilant residues, or antimicrobial agents, can damage mammalian cells in vitro. Under the ISO 10993-1 risk management framework, dressings are classified as surface medical devices contacting breached or compromised skin for limited, prolonged, or long-term durations, and in vitro cytotoxicity is a baseline test for essentially all such devices regardless of contact duration.

The primary standard is ISO 10993-5 (Biological evaluation of medical devices — Part 5: Tests for in vitro cytotoxicity), supported by ISO 10993-12 for sample preparation and reference materials. In China, the equivalent standard GB/T 16886.5 applies to NMPA submissions. The test is typically performed using established cell lines such as L929 mouse fibroblasts, BALB/3T3 cells, or V79 cells, and the outcome is judged either qualitatively by microscopic grading of cell morphology on a 0–4 reactivity scale or quantitatively by measuring cell viability relative to a blank control.

According to ISO 10993-5, a reduction in cell viability to below 70% of the blank control indicates a cytotoxic potential, and for qualitative assays a reactivity grade greater than 2 is generally considered unacceptable. Test systems must include a negative control (such as high-density polyethylene reference material), a positive control (such as natural rubber latex or a ZDEC-containing reference material), and a blank control of the extraction medium to demonstrate system suitability.

Testing Samples

Absorbent Fiber Dressings

Gauze, non-woven, cotton, alginate, and carboxymethyl cellulose (CMC) fiber dressings are absorbent materials. Their extraction is typically performed at a ratio of 0.1 g of material per mL of extraction medium, reflecting the standardized ratio assigned to absorbents in ISO 10993-12.

Foam and Hydrocolloid Dressings

Polyurethane and silicone foam dressings and hydrocolloid dressings containing CMC, gelatin, and elastomeric adhesives are tested as final cured materials. Adhesive layers and wound-contact surfaces should be represented in the test sample because they directly interface with the wound bed.

Hydrogel and Film Dressings

Hydrogel dressings may contain humectants, preservatives, and water-soluble polymers that can influence the osmolality and pH of extracts, and transparent film dressings are thin polymeric membranes for which a surface-area-based extraction ratio of 3 cm²/mL or 6 cm²/mL is applied.

Antimicrobial and Bioactive Dressings

Dressings incorporating silver, iodine, polyhexamethylene biguanide (PHMB), medical-grade honey, zinc, or chitosan are frequently the most challenging category, because their intended active release can itself produce cytotoxic responses in vitro. Method selection and result interpretation for these products must account for the intended clinical release of active substances.

Representative Coupons and Components

Where the finished device is too large or irregular, representative coupons of all patient-contacting materials, including substrates, adhesives, coatings, release liners, and sterilized configurations, may be tested individually or as a composite to mimic clinical exposure.

Testing Items

Typical cytotoxicity testing items for medical dressings include:

• Qualitative in vitro cytotoxicity by extract, direct contact, or indirect contact method with morphological grading of reactivity (grades 0 to 4).
• Quantitative in vitro cytotoxicity using viability endpoints such as the MTT tetrazolium reduction assay, neutral red uptake, or water-soluble tetrazolium salts (WST-1, WST-8/CCK-8), reported as percentage viability versus blank control.
• Serial dilution testing of extracts (for example, 100%, 50%, 25% concentrations) to establish a dose–response relationship for products with borderline results.
• Comparative cytotoxicity assessment of design variants, raw material suppliers, sterilization methods, or packaging configurations.
• Supporting characterization such as extractable pH, osmolality, and visual observations of turbidity or precipitation, which aid in interpreting assay results.

Testing Methods

Sample Preparation and Extraction

Extraction is performed under aseptic conditions using culture medium such as MEM or DMEM supplemented with serum (the preferred polar medium for extract testing) or other media justified by the material. Standard extraction conditions from ISO 10993-12 include 37°C for 24 hours, 37°C for 72 hours, 50°C for 72 hours, 70°C for 24 hours, or 121°C for 1 hour; the 37°C/24-hour condition is most common for protein-based or thermally sensitive dressings. Extraction ratios are selected based on material geometry: 0.1 g/mL for absorbents, mass-based ratios for elastomers and composites, and 3 cm²/mL or 6 cm²/mL for film-type materials.

Extract (Elution) Method

Cells are seeded in multi-well plates and exposed to the test extract for 24 hours (typically 24 ± 2 hours) at 37°C in a humidified incubator with 5% CO2. Viability is then quantified, most commonly by the MTT assay measured at 570 nm, and calculated as a percentage of the blank control. This method is the default choice for most dressings because it captures the cumulative effect of all leachable substances and can accommodate both polar and non-polar extraction media.

Direct Contact Method

A sterile test specimen is placed directly onto a sub-confluent cell monolayer and incubated for 24 ± 2 hours. Cells are then examined microscopically for morphological changes, lysis, and zone-of-inhibition effects around and beneath the specimen, graded on the ISO 10993-5 scale of 0 (none) to 4 (severe). The method suits low-density materials and detects toxicity from both leachables and physical surface interaction, but sample mass and geometry must be controlled to avoid mechanical artifacts.

Indirect Contact Methods

In the agar diffusion (agar overlay) method, cells are covered with a thin layer of nutrient agar before the specimen is placed on the agar surface, so that diffusible toxic substances migrate to the cells while physical contact is prevented. This method is useful for dressings with antimicrobial or eluting components and for high-density materials. The filter diffusion method, in which cells grown on a cellulose acetate filter are exposed to material extractables diffusing through the filter, provides an alternative indirect format.

Acceptance Criteria

For quantitative assays, cell viability of at least 70% of the blank control satisfies the ISO 10993-5 acceptance criterion, while results below 70% indicate cytotoxic potential requiring further investigation. For qualitative assays, reactivity grades of 0, 1, or 2 are generally acceptable, and grades of 3 or 4 indicate a cytotoxic effect. Positive and negative controls must respond within laboratory-established limits for a test to be considered valid.

Applications

Cytotoxicity testing data for medical dressings are used across the product life cycle, including:

• Regulatory submissions, including NMPA registration in China, CE technical documentation under the EU MDR 2017/745, and FDA 510(k) submissions, where a biological evaluation report per ISO 10993-1 must document the cytotoxicity assessment.
• Biological evaluation programs that combine cytotoxicity with irritation and sensitization testing (ISO 10993-10), and where indicated, additional endpoints such as material-mediated pyrogenicity or subacute toxicity.
• Research and development screening of new materials, adhesive formulations, hydrogel polymers, and antimicrobial loadings before committing to full biocompatibility panels.
• Change control, evaluating the biological impact of changes to raw material suppliers, manufacturing processes, sterilization method, or primary packaging.
• Verification of sterilization processes by confirming that residues such as ethylene oxide and its reaction products do not contribute to cytotoxic effects.
• Quality monitoring performed by accredited third-party laboratories or in-house facilities operating under GLP or ISO/IEC 17025-accredited quality systems.

FAQ

Which standard governs cytotoxicity testing of medical dressings?

ISO 10993-5 specifies the test methods and acceptance criteria, while ISO 10993-12 governs sample preparation, extraction ratios, and reference control materials. GB/T 16886.5 is the corresponding Chinese national standard accepted for NMPA submissions.

Which method should be selected for a dressing product?

The extract (elution) method with quantitative MTT readout is the default for most dressings because it evaluates the combined effect of all leachables. Direct contact is useful for soft, low-density materials, and agar diffusion is appropriate for eluting or antimicrobial products where diffusion behavior is of interest. The rationale for method selection must be documented in the biological evaluation.

What causes dressings to fail cytotoxicity testing?

Common causes include residual monomers, catalysts, adhesive tackifiers, plasticizers, preservatives, ethylene oxide sterilant residues, and intentionally released antimicrobials such as silver or iodine ions. Abnormal extract pH or osmolality, particularly in hydrogels, can also depress viability readings and should be checked during interpretation.

How many samples are required and how long does testing take?

Sample requirements depend on the extraction ratio and the number of test conditions; as a practical guide, 5 to 20 grams or an equivalent surface area of finished product per condition is typically sufficient. The laboratory phase usually takes approximately two to four weeks, including cell culture, extraction, exposure, and reporting, though turnaround varies with method and laboratory workload.

Does a passing cytotoxicity result complete the biological evaluation?

No. Cytotoxicity is one endpoint within the ISO 10993-1 evaluation plan. Dressings contacting breached skin generally also require irritation and sensitization data, and additional endpoints may be added based on contact nature, duration, and the risk assessment of materials.