Performance of Polymeric Skin Adhesives during Perspiration

This short review article is based on the research paper titled “Performance of polymeric skin adhesives during perspiration”, authored by Daniel Hansen, Saeed Zajforoushan Moghaddam, Johannes Eiler, Kristoffer Hansen and Esben Thormann.

The research paper was published in ACS Applied Polymer Materials journal in March, 2020. The study explores the performance of three polymeric skin adhesives with similar rheological properties but different ability to absorb artificial sweat. To evaluate the different polymer materials, a setup was used for probing peel adhesion of adhesives under realistic wear conditions.

A flow sensor and a flow reader were used to measure the artificial sweating rate.

ABSTRACT

Probing the performance of skin adhesives is of great interest for rational material formulation as these polymer materials are used for attaching medical devices to the skin. 

In this study, the authors found that sweat introduced at the substrate−adhesive interface limits further bonding of the adhesives by restricting viscous flow. The authors also demonstrated that water-absorbing skin adhesives have higher peel forces compared to non absorbing adhesives under sweating conditions.

To explore this further , a setup for probing peel adhesion of adhesives was used under realistic wear conditions. A perspiration simulator, which includes a skin mimicking gelatin substrate with controlled roughness and the ability to perspire with a tunable sweat rate. The sweat rate was measured via a microfluidic flow sensor and a flow reader provided by Elveflow.

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INTRODUCTION

Because skin adhesives represent a class of polymer composites used for attaching medical devices, they must be easy to remove without damaging the skin while providing sufficient adhesion to ensure optimal functionality of the attached device [1].

On one hand, the presence of sweat can compromise adhesion by disturbing nonspecific interactions between the adhesive and skin [2]. On the other hand, prolonged exposure to sweat accumulated between the skin and adhesive can cause skin maceration [3,4].

Standard clinical investigations where adhesive performance during wear is evaluated are often costly and require a large number of subjects to yield results of statistical significance due to large intra- and intersubject variation [5].

As a consequence, developing skin models for in vitro adhesive testing is of a growing demand. These skin models would mimic relevant skin parameters, such as surface roughness, mechanical properties, hydration, and surface energy [6−9].

In this work, the authors present a new perspiration simulator having dimensions appropriate for conventional adhesive peel experiments [10]. In the setup, a flow sensor and a flow reader were used to follow up the sweat rate.

Adhesive formulations were evaluated as a function of application pressure prior to artificial perspiration and different perspiration times to systematically investigate the link between adhesive bonding, amount of artificial sweat, water transport, and peel adhesion.

AIM & OBJECTIVES

 In short, this paper is about :

• the development of a new perspiration simulator for evaluating the performance of skin adhesives during realistic wear conditions.
• the evaluation of three skin adhesive formulations with indistinguishable rheological properties, but different abilities to absorb the artificial sweat, by measuring the peel adhesion directly from the perspiration simulator.

MATERIALS & METHODS

The artificial skin is a multilayer substrate, where each layer holds a functionality. The skin includes a track-etched membrane. The track-etched membrane was laminated to the polyimide support by using a double-sided acrylic adhesive. The acrylic adhesive was applied to the polyimide support prior to drilling the artificial sweat pores to ensure that the adhesive doesn’t block the pores. The track-etched membrane provides high flow resistance, which yields a pressure drop much larger than the differences in Laplace pressure of the holes in the polyimide support, which ensures homogeneous sweating throughout the substrate [11].

The polyimide support gives mechanical stability and large scale roughness. The polyimide support was coated with gelatin (Figure 1) through a dip-coating procedure. To mimic realistic wear conditions, the artificial skin was clamped to a reservoir, which was connected to a microfluidic flow sensor and a tank supplying the artificial sweat solution. 

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KEY FINDINGS

In this study, the authors evaluated 3 adhesives with indistinguishable rheological properties but different ability to absorb artificial sweat. The rheological properties were fixed to decouple the bulk mechanical properties from events occurring at the substrate−adhesive interface. The effects of application pressure, dwell time, and perspiration were quantified for each adhesive formulation.

The results suggest that some of the initial substrate−adhesive contact area is compromised due to the pressure applied from the artificial sweat glands (∼4.5 kPa), which could be a result of the occlusive nature of the Starch adhesive. The CMC and CMC−Starch adhesives are less occlusive through their water absorption and, in consequence, do not show this significant drop in peel force.

For the low application pressure (Figure 3), the peel forces after 20 min do not reduce below the initial (0min) peel forces, which indicates that the initial substrate−adhesive contact area remains throughout the perspiration process.

Figure 4 indicates that having hydrophilic fillers near the skin−adhesive interface to absorb sweat maintains the adhesive properties during perspiration. The hydrophilic CMC fillers absorb sweat at the substrate−adhesive interface, which allows the skin adhesive to flow and bond to the substrate and, in result, increase the substrate−adhesive contact area over time. The CMC−Starch adhesive appears to manifest peel forces in between the CMC and Starch adhesive as expected from its capacity for water absorption, which also is in between the two other adhesives.

CONCLUSION

The authors present a perspiration simulator, which includes a skin mimicking gelatin substrate with controlled roughness and the ability to perspire with a tunable sweat rate. To measure the sweat rate, a flow sensor and a flow reader were used. 

Artificial sweat introduced at the artificial skin−adhesive interface proved to reduce viscous flow of the adhesives and consequently limited the adhesive’s ability to bond to the substrate. However, highly water-absorbing adhesives were found to bond under perspiration conditions depending on the total amount of artificial sweat introduced. In conclusion, the absorbing adhesive showed a significant increase in peel adhesion compared to non-sweat-absorbing adhesives.

If you’re interested in reproducing what the authors have done, Elveflow also provides a pressure-driven flow controller that allows the operator to finely control the flow rate of various fluids. For any additional information, feel free to contact our team of experts!

References