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ISO 28821:2012 — Medical devices — Needle-based injection systems — Test methods
Standardised laboratory test methods for verifying the performance of needle-based injection systems
ISO 28821:2012 specifies the test methods for verifying the performance of needle-based injection systems defined in ISO 28820. It provides standardised measurement procedures for dose accuracy, injection force, needle insertion characteristics, and safety feature performance across all categories of needle-based injectors.
1. Dose Accuracy Measurement — Gravimetric and Volumetric Methods
ISO 28821 defines the reference test method for dose accuracy as gravimetric measurement using a precision balance with a resolution of at least 0.1 mg and accuracy of ±0.5 mg. The test involves dispensing the device’s nominal dose into a tare-weighted container, measuring the mass of expelled liquid, and converting to volume using the density of the test liquid (typically deionised water at 20 °C with density 0.9982 g/mL). For each configuration and operating condition, a minimum of 10 consecutive measurements must be performed, and the mean delivered volume must fall within the tolerance limits defined by ISO 28820.
The standard addresses several critical measurement artefacts that can affect dose accuracy results. Air bubbles trapped in the syringe barrel or needle hub reduce the delivered volume and must be rigorously eliminated before testing through a standardised priming procedure (five full-stroke purges followed by visual inspection). Needle tip wetting — the phenomenon where liquid remaining on the external needle surface after dispensing is not captured in the gravimetric reading — can introduce a systematic negative bias estimated at 0.5–2 μL for typical 26G needles. The standard recommends a correction factor of 1.5 μL per dispensing cycle for needles ≤ 27G to compensate for tip wetting losses.
| Parameter | Test Condition | Acceptance Criterion | Measurement Uncertainty |
|---|---|---|---|
| Primary dose accuracy | 20 °C ± 2 °C, 50 % ± 10 % RH | ±5 % (≥ 1 mL), ±10 % (< 1 mL) | ≤ 1 % of reading |
| First-dose accuracy | After 7-day storage | Same as primary | ≤ 1 % of reading |
| Last-dose accuracy (multi-dose) | 90 % of nominal capacity | Same as primary | ≤ 1 % of reading |
| Temperature extreme | 5 °C and 40 °C | ±10 % (all doses) | ≤ 2 % of reading |
| Altitude simulation | Equivalent to 3000 m | ±15 % | ≤ 3 % of reading |
When testing autoinjectors with spring-driven delivery, be aware that the dose accuracy can vary significantly with the device orientation — vertical (needle-up vs. needle-down) and horizontal positions must all be tested separately. The air bubble migration dynamics differ between orientations, and the spring force may be affected by gravitational effects on internal components. ISO 28821 requires full factorial testing across all clinically relevant orientations.
2. Injection Force Measurement and Glide Force Characterisation
ISO 28821 specifies two distinct force measurements for needle-based injection systems: break-loose force (the peak force required to initiate plunger movement) and dynamic glide force (the sustained force required to maintain plunger movement at a specified rate). The test uses a universal testing machine or dedicated force-displacement test stand fitted with a load cell (capacity 100 N, accuracy ±0.5 % of reading) and a displacement transducer (resolution 0.1 mm). The crosshead speed is set at 100 mm/min for manual syringes and at the device-specific actuation speed for power-driven devices.
For autoinjectors and pen injectors, the standard adds a third measurement: activation force, defined as the force required to trigger the device’s automatic needle insertion and drug delivery sequence. The activation force must be high enough to prevent accidental triggering during handling (typically ≥ 15 N) but low enough to be achievable by the target user population (typically ≤ 50 N for patients with reduced hand strength). The force-displacement curve must be recorded and analysed for anomalies such as stick-slip behaviour, which indicates inadequate lubrication or an incompatible stopper-barrel material pairing.
Glide force consistency is a strong indicator of manufacturing quality. A well-controlled injection device should show a coefficient of variation (CV) of less than 10 % in dynamic glide force across a production batch. When your CV exceeds 15 %, investigate potential root causes: stopper dimensional variability, barrel inner diameter uniformity, lubricant distribution evenness, or assembly process parameters.
| Force Parameter | Test Speed | Typical Range (1 mL syringe) | Common Failure Mode |
|---|---|---|---|
| Break-loose force | 10 mm/min pre-test speed | 8–30 N | Excessive stiction after storage |
| Dynamic glide force (manual) | 100 mm/min | 3–15 N | Stick-slip causing dose inaccuracy |
| Activation force (autoinjector) | Device-specific | 15–50 N | Too high for elderly users |
| Needle insertion force | 10 mm/s | 0.3–1.2 N (26G) | Blunt needle causing pain and tissue trauma |
| Plunger rod buckling | At max insertion speed | > 50 N critical load | Buckling leads to incomplete dose delivery |
3. Safety Feature Performance Testing
ISO 28821 dedicates a substantial section to the test methods for safety features, particularly those designed to prevent needlestick injuries and reuse. The standard defines tests for needle shield activation force, shield locking mechanism integrity, and single-use lockout mechanisms. For passive safety syringes where the needle shield deploys automatically after complete plunger depression, the test must verify that the shield covers the needle tip within 0.5 seconds of injection completion and that the locking mechanism withstands a minimum pull-off force of 10 N without dislodgement.
Reuse prevention features are tested through a sequence of simulated reuse attempts. For devices with a plunger break-away mechanism (where the plunger fractures after use), the test applies a compressive force of 50 N to simulate attempted reuse — the mechanism must prevent drug re-aspiration or further dispensing. The standard also defines a sharpness test method for safety needles: after activation of the safety feature, the shielded needle tip must not be capable of penetrating a standardised latex membrane under a contact force of 2 N, simulating accidental finger contact during disposal.
One of the most frequently cited failures in safety syringe post-market surveillance is the “partial activation” phenomenon — where the safety shield deploys only partially, leaving the needle tip exposed. ISO 28821 requires that the safety feature be tested after injection at various actuation speeds (fast, normal, and slow), and that a clear audible or tactile confirmation of full deployment be provided to the user. Visual confirmation alone is insufficient for devices used in suboptimal lighting conditions.
Frequently Asked Questions
Q1: Can ISO 28821 tests be performed with the actual drug product instead of water?
Yes. While the standard defines water as the reference test liquid for comparability, the manufacturer should also perform verification testing with the actual drug formulation, particularly for high-viscosity biologics (> 20 cP) where the injection force can increase by 3–5× compared to water and the dose accuracy may be affected by the non-Newtonian flow characteristics.
Q2: How is needle penetration force measured?
The standard references ISO 7864 for the needle penetration force test method. A sharpness tester uses a specialised fixture with a polymer membrane (polyurethane or silicone) of controlled thickness and hardness. The needle is driven through the membrane at a specified speed (10 mm/s), and the peak force is recorded. For a 26G hypodermic needle, the maximum allowable penetration force is 0.5 N.
Q3: What statistical methods are specified for data analysis?
The standard recommends calculating the mean, standard deviation, and 95 % confidence interval for all measured parameters. For dose accuracy testing, the individual delivered volumes must be reported along with the mean and the percentage deviation from nominal. Tolerance intervals covering 95 % of the population with 95 % confidence (95/95 tolerance) are recommended for process validation studies.
Q4: How is the test environment controlled?
All tests must be conducted in a controlled environment at 20 °C ± 2 °C and 50 % ± 10 % relative humidity. Devices must be preconditioned at the test environment for at least 4 hours before testing. For temperature-extreme testing, the devices must be soaked at the target temperature for a minimum of 2 hours, and the test must be completed within 60 seconds of removal from the conditioning chamber to minimise temperature drift effects.
