Testing standard
Standard Test Method for Strength Properties of Tissue Adhesives in T-Peel by Tension Loading
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM F2256 bonds two strips of soft tissue substrate — normally frozen or split pig skin — with the tissue adhesive under test and peels them apart in a T configuration under position control. It exists to compare surgical adhesives and sealants with one another, not to produce absolute design values. The current edition is ASTM F2256-24.
Two strips of soft tissue substrate are bonded along part of their length with the tissue adhesive under test. The unbonded ends are bent apart to form a T, clamped in the machine, and drawn apart under position control so the bond is peeled open progressively from one end. The force through the separation is recorded.
The substrate is the unusual part. Tissue adhesives are intended to bond living soft tissue, so the specimens are grafts of frozen or split pig skin rather than metal or plastic coupons. The adhesive is being tested against something that behaves like what it will be used on.
Surgical adhesives and sealants close wounds and seal openings against leakage. What matters clinically is whether the bond survives the movement, the wetness and the pressure of a real site — and peel is the loading that finds a weak bond fastest, because it concentrates the whole force on a moving line rather than spreading it over the bonded area.
The standard's stated purpose is comparison: it provides a means for comparison of the adhesive strengths of tissue adhesives intended for use as surgical adhesives or sealants, or both, on soft tissue. With an appropriate choice of substrate it can also be used for quality control in the manufacture of tissue-adhesive-based devices.
Comparison is the right word for it. Pig skin varies from animal to animal and from site to site on the same animal, and a T-peel figure on tissue carries more scatter than the same test on an engineered substrate. The method is a ranking and screening tool, not a source of absolute design values, and treating it as the latter is the main way it is misused.
Testing an adhesive against tissue rather than against metal is what makes the result relevant, and it is also what makes it variable.
Average peel force over the bonded length, per unit width
An average, not a peak. A peak on a tissue substrate is as likely to be a feature of the skin as of the adhesive.
Peel concentrates the load on a moving line
It is the loading that finds a weak bond fastest, which is what a screening method wants.
The stated purpose is comparison between adhesives
Biological substrate variability is large. Using a T-peel figure as an absolute design strength is the main way this method is misused.
Light jaws with a soft face, holding wet slippery skin without crushing it or letting it slide. Clamping force is set and recorded rather than left to the operator, because both errors move the result and neither is obvious in the trace.
SpecificationsPeel forces here are commonly a few newtons or less. A cell sized in newtons is not a refinement on this method — it is the difference between a measurement and a flat line.
SpecificationsForces are very low — commonly a few newtons or less — so a small frame with a load cell sized in newtons and force accuracy to ASTM E4 over that range is essential. Resolution, not capacity, is the whole requirement.
The test is position controlled, so a machine that holds a steady low crosshead speed is needed, together with enough travel to peel the full bonded length in one pass.
Grips have to hold wet, slippery pig skin without crushing it or letting it slide. Light pneumatic jaws with a soft face are the usual answer, and clamping force is set rather than guessed: too much and the substrate is damaged at the jaw, too little and the specimen creeps out during the peel.
Where the specimen is tested wet at body temperature, a heated bath or an environmental enclosure around the fixture is needed, and that is separate equipment from the frame.
Substrate variability dominates and is managed by specimen count rather than eliminated. Skin allowed to dry between preparation and test stiffens and reads differently. Specimens that slip in the jaws add travel that looks like peel. And a peak force reported instead of an average over the peel exaggerates the bond, on a test where the average is the meaningful quantity.
Three ASTM methods load a tissue adhesive bond three ways, and a product is normally characterised on more than one.
| T-peel (F2256) | Tension (F2258) | Lap shear (F2255) | |
|---|---|---|---|
| Loading | Progressive peel from one end | Straight tension across the bond | Shear along the bond |
| Finds | A weak bond fastest | Bulk bond strength | Resistance to sliding |
| Substrate | Soft tissue | Soft tissue | Soft tissue |
| Force needed | Very low | Low | Low |
| Clinically resembles | A wound edge lifting | A sealed opening under pressure | Tissue planes sliding |
All three carry the variability of a biological substrate. They rank adhesives against one another reliably and they do not produce absolute strengths that can be carried into a design calculation.
ASTM F2256 is the standard test method for the strength properties of tissue adhesives in T-peel by tension loading. Two strips of soft tissue substrate are bonded with the adhesive under test, the unbonded ends are bent apart to form a T, and they are drawn apart under position control while the force is recorded. The current edition is ASTM F2256-24, which revised F2256-05(2015).
Because the adhesive is intended to bond living soft tissue, and a bond to steel or plastic tells you very little about that. Frozen or split pig skin behaves closely enough to human soft tissue to make the comparison meaningful. The trade-off is variability: skin differs between animals and between sites on one animal, and that scatter is inherent to the method rather than a sign of poor technique.
Comparison. The standard states its purpose as providing a means for comparison of the adhesive strengths of tissue adhesives intended for use as surgical adhesives or sealants on soft tissue, and notes that with an appropriate choice of substrate it may also be used for quality control in manufacturing tissue-adhesive-based devices. It ranks products against one another; it does not produce absolute strengths for a design calculation, and treating it as though it does is the commonest misuse.
Because peel is the harshest loading and therefore the most efficient screen. A T-peel concentrates the whole force on a moving line at the edge of the bond rather than spreading it over the bonded area, so a weak or uneven bond shows up quickly and at a low force. Tension and lap shear methods exist alongside it — F2258 and F2255 — and a product is normally characterised on more than one.
For most products, yes. Surgical adhesives are applied to tissue that is wet and often bleeding, and a bond formed and tested dry is not the bond the surgeon gets. Where the product is intended for a wet field the specimens are conditioned and tested wet, and where body-temperature performance matters a heated bath or enclosure around the fixture is needed. The condition is reported either way, because it moves the result substantially.
A small frame with a load cell sized in newtons and force accuracy to ASTM E4 over that range — peel forces are commonly a few newtons or less, so resolution is the entire requirement. The test is position controlled, so a machine that holds a steady low crosshead speed is needed, with enough travel to peel the full bonded length in one pass. Grips have to hold wet skin without crushing it.
By specimen count and by consistent substrate preparation, not by trying to eliminate it. Skin thickness, site and hydration all contribute, so laboratories fix what they can — one animal batch, one anatomical site, one preparation method, one hydration protocol — and then run enough specimens that the comparison between adhesives is bigger than the noise. A report giving a mean without a spread and a specimen count is not usable.
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | Very low — commonly a few newtons or less | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ASTM E4 over the working range | Verified to ASTM E4, and to ISO 7500-1 Class 0.5 |
| Gripping | Light pneumatic jaws with a soft face, holding wet skin without crushing it or letting it slide | Our peel and adhesion fixtures, built to the specimen |
| Environment | Tested wet where the product is intended for a wet field, and at body temperature where that matters, which needs a heated bath around the fixture | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.
