Hemolysis rate testing is an essential biocompatibility study for silver-containing collagen wound dressings that contact blood and wound exudate during clinical use. Performed in accordance with ISO 10993-4, GB/T 16886.4, and ASTM F756, the assay quantifies the extent of red blood cell membrane damage caused by the dressing and its leachable substances, particularly silver ions. This article explains the sample preparation, direct and indirect test methods, calculation formula, acceptance criteria, and regulatory applications of the hemolysis test for advanced antimicrobial wound care products.
Technical Overview
Silver-containing collagen wound dressings combine the natural extracellular-matrix support of collagen with the broad-spectrum antimicrobial activity of silver ions or silver-based compounds such as silver sulfadiazine or silver nanoparticles. Although these dressings are applied to wound surfaces rather than the intravascular compartment, they routinely contact blood and protein-rich exudate in acute and chronic wounds, including diabetic foot ulcers, pressure injuries, burns, and surgical sites. Because released silver ions are known to interact with cell membranes and proteins, hemocompatibility must be demonstrated before market authorization.
The hemolysis rate experiment is the primary screening test for blood compatibility of external-use devices that may contact circulating blood. The test measures the percentage of erythrocytes that rupture when exposed to the test article or to an extract of the test article, compared with fully lytic (positive control) and non-lytic (negative control) conditions. The study is typically conducted by accredited third-party laboratories or in-house biocompatibility facilities following ISO 10993-4 (GB/T 16886.4) for the selection of blood-interaction tests, ISO 10993-12 (GB/T 16886.12) for sample preparation and extraction, and ASTM F756 for the assessment of hemolytic properties of materials.
A hemolysis rate of less than 5% is the widely accepted criterion for classifying a material as non-hemolytic. For silver-containing collagen dressings, the test is especially informative because it directly reveals whether the silver-release profile of the formulation damages erythrocyte membranes under simulated clinical exposure conditions.
Testing Samples
Finished Sterile Dressing (Direct Contact Samples)
Samples are taken from the final sterilized product in its commercial packaging, as silver speciation and collagen integrity can be affected by terminal sterilization methods such as ethylene oxide or gamma irradiation. Representative pieces are cut from multiple locations of the dressing to account for possible non-uniform silver loading. Sample mass or surface area is documented and the extraction ratio is defined according to ISO 10993-12.
Test Extracts (Indirect Contact Samples)
Extracts are prepared by incubating the dressing in 0.9% sodium chloride injection or phosphate-buffered saline at 37°C for 24 hours using a standard ratio such as 0.2 g/mL of material to extraction medium. Where a worse-case evaluation is required, exaggerated extraction conditions, for example 50°C for 72 hours, may also be applied. The extract captures the leachable fraction of the material, including dissolved silver species, low-molecular-weight collagen degradation peptides, and residual processing aids.
Control Materials
Each run includes a positive control, typically sterile water for injection or distilled water, which causes complete hemolysis, and a negative control, typically 0.9% sodium chloride injection or PBS, which should produce negligible hemolysis. A reference material with a known hemolytic profile may also be included to monitor method sensitivity, and a silver-free collagen dressing is frequently run in parallel to isolate the contribution of the silver component.
Testing Items
The core study design for silver-containing collagen wound dressings includes the following test items:
- Hemolysis rate by the direct contact method, evaluating the physical and chemical effects of the intact dressing surface on diluted blood.
- Hemolysis rate by the indirect extraction method, evaluating the effect of leachable substances, most importantly released silver ions.
- Absorbance of the supernatant at 545 nm, the characteristic wavelength for hemoglobin quantification.
- Validation of positive and negative controls, confirming that the positive control produces complete hemolysis and the negative control remains essentially non-hemolytic.
- Optional supporting measurements, such as silver ion concentration in the extract by ICP-MS or ICP-OES, to correlate silver release with the observed hemolytic response.
Testing Methods
Blood Collection and Preparation
Fresh blood is obtained from healthy New Zealand white rabbits or healthy human donors in accordance with the applicable ethical protocol. The blood is anticoagulated with heparin or with 3.8% trisodium citrate at a ratio of 1:9 anticoagulant to blood, and it must be used within a defined short period, typically a few hours, to preserve erythrocyte integrity. Immediately before the test, the blood is diluted, for example 8 mL of whole blood added to 10 mL of 0.9% sodium chloride solution, and gently inverted to ensure homogeneity without mechanical shear.
Direct Contact Hemolysis Test
Pre-weighed or measured dressing specimens are placed into tubes containing a fixed volume of pre-warmed negative control medium and equilibrated at 37°C. A defined volume of diluted blood, commonly 0.2 mL, is added to each tube, and the tubes are incubated at 37°C ± 1°C for 60 minutes with gentle inversion at intervals to maintain contact between the blood and the sample. The test is performed in triplicate for each sample and for each control.
Indirect Contact (Extraction) Hemolysis Test
The prepared extract is pre-warmed to 37°C, and aliquots are dispensed into test tubes. Diluted blood is added in the same manner as for the direct method, and the tubes are incubated at 37°C ± 1°C for 60 minutes. This approach evaluates only the soluble leachables of the dressing and is particularly relevant for assessing the contribution of silver ions, which diffuse readily into the extraction medium.
Centrifugation and Absorbance Measurement
After incubation, all tubes are centrifuged at approximately 2500 rpm for 5 minutes to sediment intact erythrocytes. The supernatant is carefully transferred to clean cuvettes, and the absorbance of released free hemoglobin is measured spectrophotometrically at 545 nm against the negative control medium as the blank. Gross observation for hemolysis or erythrocyte agglutination and coagulation is also recorded.
Calculation of Hemolysis Rate
The hemolysis rate of each sample is calculated from the mean absorbance values of the triplicate tubes using the following formula:
Hemolysis Rate (%) = [(A_sample − A_negative) / (A_positive − A_negative)] × 100%, where A_sample is the absorbance of the test article tube, A_negative is the absorbance of the negative control tube, and A_positive is the absorbance of the positive control tube. Results are reported as the mean ± standard deviation, and the test is considered valid only when the control values fall within the predefined acceptance window.
Acceptance Criteria and Interpretation
A hemolysis rate below 5% indicates that the material is non-hemolytic and acceptable for the intended wound-contact application. A rate of 5% or greater classifies the material as potentially hemolytic and triggers formulation review, typically focusing on the silver loading, the silver release kinetics, or the type of silver compound used. If the indirect method passes but the direct method fails, the response may relate to surface interactions or adsorption phenomena; conversely, a high indirect result with a passing direct result points to problematic leachables. Either outcome should be cross-checked against cytotoxicity data from ISO 10993-5 and against the clinical exposure scenario defined in the risk management file per ISO 10993-1.
Applications
Hemolysis rate data for silver-containing collagen wound dressings serve multiple purposes across the product lifecycle:
- Regulatory registration dossiers for Class II or Class III medical devices in markets such as China (NMPA), the European Union (CE marking under the MDR), and the United States (FDA 510(k) submissions), where hemocompatibility evidence is required for devices contacting compromised skin and potentially blood-borne exudate.
- Research and development screening, in which candidate silver compounds, carrier matrices, and loading levels are compared to identify formulations that maintain antimicrobial efficacy while remaining non-hemolytic.
- Design change verification, confirming that modifications to collagen source, crosslinking agents, or silver deposition processes do not adversely affect blood compatibility.
- Batch consistency evaluation and stability studies, in which aged or terminally sterilized product lots are re-tested to confirm that silver speciation changes over shelf life do not introduce hemolytic risk.
- Integration into the biological risk assessment and evaluation plan, complementing cytotoxicity (ISO 10993-5), sensitization and irritation (ISO 10993-10 and ISO 10993-23), and implantation or systemic toxicity studies as required by ISO 10993-1.
FAQ
What is the pass criterion for the hemolysis rate test? A hemolysis rate below 5% is the generally accepted pass criterion, consistent with the principles of ASTM F756, ISO 10993-4, and GB/T 16886.4. Values of 5% or above indicate a hemolytic potential that requires further investigation or design modification.
Why is hemolysis testing especially important for silver-containing dressings? Silver ions exert their antimicrobial effect by interacting with bacterial membranes and proteins, and these same mechanisms can, at sufficiently high concentrations, disrupt erythrocyte membranes. The hemolysis test therefore provides a direct, sensitive indicator of whether the silver-release level of the dressing stays within a biocompatible window.
Which method should be chosen, direct contact or extraction? The two methods answer complementary questions. The direct contact method evaluates the combined effect of the dressing surface and released substances on intact blood, while the extraction method isolates the effect of leachables. For a comprehensive dossier, particularly for regulatory submission, both are frequently performed.
What type of blood is used in the experiment? Fresh anticoagulated rabbit blood is most commonly used, especially for studies aligned with GB/T 16886.4 practice, while human donor blood may be used under protocols aligned with ASTM F756. The blood must be fresh, handled gently, and diluted consistently, because aged or mechanically damaged blood can produce falsely elevated baseline hemolysis.
Where should the test be performed? The test should be performed in laboratories with documented competence in medical device biocompatibility testing, such as accredited third-party laboratories operating under ISO/IEC 17025 quality systems, ensuring that the generated data will be accepted by regulatory authorities.
How does the hemolysis result relate to other biocompatibility endpoints? Hemolysis is one element of the ISO 10993 evaluation set. A non-hemolytic result supports, but does not replace, cytotoxicity, sensitization, irritation, and systemic toxicity data. Regulators expect the complete biological evaluation plan to be justified in the risk management file for the specific contact duration and contact category of the dressing.
