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Products that adhere to the skin, such as plasters and tapes, are special in clinical development because of their adhesive properties. In addition to the aspects of skin compatibility and efficacy, the investigation of the product's adhesive properties is a third dimension of investigation that does not exist in this form for other products. 

We interviewed Ms Frederike Falk, head of our study site in Elmshorn, to shed more light on the background and procedure of adhesive strength studies.

Adhesive testing

Interviewer: Hello Frederike! Our topic today is adhesive strength measurement. What's the use of that?

Frederike: Customers want to know how well their adhesive dressings stick. We test this as material test under laboratory conditions, everyday conditions, and even special ones, like when someone's sweating. This is crucial when developing adhesive dressings, wound dressings, and other adhesive products.

Interviewer: Can you break down what that means and how it's done?

Frederike: The classic adhesive strength test is about measuring how much force is needed to peel off an adhesive dressing. Typically, we stick strips onto a person's back. These strips are usually between 1.5 and 2.5 centimeters wide. Depending on their size, we can fit up to 20 strips—10 on the upper back and 10 on the lower. Each product is applied twice, once on the upper part and once on the lower. They're then firmly rolled on with a roller to standardize the contact pressure. 

Interviewer: What's the general procedure?

Frederike: After application, the adhesive dressings stay on for as long as the customer specifies—typically, it's either 4 or 24 hours for us. Then, the test volunteers return. We give them some time to acclimatize so they aren't sweating profusely when they arrive. After that, they sit in a special chair, and our Universal Testing Machine peels off one dressing at a time. 

Interviewer: Universal Testing Machine? Is that its real name? Could you describe it?

Frederike: Yes, it's a versatile device, hence the name. We use it for adhesive strength testing and also in our hair lab. It's a large machine with an extendable arm. At the end of this arm is a clamp attached to a spring-loaded sensor. The clamp is attached to the loose end of the adhesive dressing. The machine then lifts at a consistent speed, peeling the adhesive dressing off. The device measures and records the force needed to peel off the adhesive dressing in mN.

Essential to have standardized conditions

Interviewer: Any specifics to keep in mind when using this machine?

Frederike: Everything is geared towards achieving standardized conditions. It starts with how the volunteers sit. For adhesive testing, they sit in a sort of massage chair. If someone has a notably curved back or can't sit upright, they're excluded. They have to sit straight to ensure the correct angle for the peeling process. We aim to minimize variations: volunteers can't be sweaty, there shouldn't be any water or cream on their backs, and they can't be too hairy.

Interviewer: So, avoiding sweat and water is because it could affect the measurements?

Frederike: Exactly. Conditions should be standardized across all volunteers. On the flip side, some clients want to know how well their adhesive dressing sticks when someone is sweating. We also have a special study design for this: After the adhesive dressings are applied, volunteers don't go home but spend a defined time in a hot room set at 38°C with 30 to 40% humidity, following FDA hotroom conditions. They mustn't lean against anything. Shortly after, the adhesive dressings are peeled off. This helps assess adhesive performance in sweaty conditions—which can be essential if clients are targeting hotter regions beyond Northern Germany.

Interviewer: What else do you observe during these tests?

Frederike: We check for things like adhesive residues, whether the adhesive dressing peels off cleanly, or if it partially or fully comes off on its own. We also see if it leaves any redness, and optionally, we assess if the volunteer felt pain during removal and how intense it was. We can also measure skin moisture after removal to evaluate how much the volunteer sweated beneath it. This provides insights into the adhesive dressing's breathability. 

Further technical test designs

Interviewer: Do we also test the safety and efficacy of wound dressings in these tests?

Frederike: No, because these are purely technical measurements using lab samples of wound dressing adhesives. But we have other study designs that can be used in clinical trials for medical devices, but those are far more complex.

Interviewer: Besides adhesive strength, are there other technical test designs?

Frederike: Yes, for instance, there's a wear test. It evaluates the overall adhesive properties of a adhesive dressing—how long it sticks and its comfort level. The standard test lasts 8 days. Adhesive dressings are applied at customer-specified spots, like near joints where there's frequent movement. The volunteers then return typically on Day 3, 5, and 8 for assessments on how much of the adhesive dressing remains intact. At the end, adhesive dressings are removed, and volunteers rate the pain upon removal and overall comfort during wear. Sometimes, we introduce a challenge to see how the plaster deals with real world situations, for example taking a shower. Although adhesive areas shouldn't typically get wet, we've even had protocols where volunteers were instructed to take full baths.

Interviewer: Sounds practical!

Frederike: Exactly. We also have a test where we dye the skin blue. This checks if the top layer of skin, the stratum corneum, comes off with the adhesive dressing.  After dyeing the skin blue, the adhesive dressing is then applied over the dyed spot. After 24 hours, the adhesive dressing is peeled off, and the color intensity is measured. There's a control patch without the adhesive dressing to compare. This test allows us to see how much, if any, skin cells came off with the adhesive dressing. We've had instances where there was barely any color left after removal, and this dye is deep ink-blue.

Interviewer: What was the most interesting study on this topic recently?

Frederike: We had one where we wanted to see how many times you could reapply and remove the same exact adhesive dressing. 

Interviewer: Thank you for the interesting interview!

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