Whether used in wound care products, ostomy applications, or for adhering wearable devices to skin, advanced pressure-sensitive adhesives are being created to improve breathability, absorbency, and porosity and to deliver atraumatic removal. The objective is to provide a long-lasting bond for extended wear and a gentle patient experience, while also including all the additional properties required for a medical adhesive such as noncytotoxic, nonirritating, nonsensitizing, compatible with sterilization, 100 percent solid after curing/coating and manufacturable in industrial scale.

New Adhesive Formulations

In the last two decades, new classes of adhesives have been developed to complement the basic acrylic and rubber adhesives. Each of those developments addressed a different unmet need.

Thin hydrocolloid. (Credit: Scapa Healthcare)

Silicone gel adhesives were introduced to reduce trauma upon removal while still providing proper bonding. Silicone gels, currently widely used in wound care, do not respond well to moisture or high flex points on the body, such as knees and elbows. They are not breathable, which leads to additional process stress in the conversion of the product and raises the cost of the adhesive. In the last five years, high tack silicone gels have been developed, delivering benefits of improved wear time as well as possibly reducing the coat weight, and therefore the cost.

Polyurethane gels have been introduced in an attempt to reduce trauma upon removal while still providing proper bonding and high breathability. However, due to the manufacturing challenge of the chemistry, less than a handful of manufacturers are able to produce polyurethane gels.

Hydrocolloid adhesives are the “body fluid resistant” alternative. They represent a special type of pressure-sensitive adhesive with both fast adhering characteristics as well as fluid absorbency. These adhesives have become key components for ostomy accessories and wound care applications. While hydrocolloids have resolved the issue of resistance to body fluid, they are generally bulky (0.3–1 mm), making them uncomfortable for users.

Furthermore, while they can manage fluid, they can be quite traumatic at removal. Today, a few hydrocolloid formulations are now available at a thickness of 0.1–0.2 mm, and an atraumatic product has been introduced.

While rubber-based adhesives have been available in a hot melt form for a long time, acrylic adhesives used to be offered as solvent or emulsion. The ability to provide hot melt acrylic and UV curable acrylic has proven to be successful. As a result, environmental and safety issues linked to the use of solvents can be eliminated, and lower adhesive coating costs can be achieved.

The Future of Adhesion Technology

Adhesives are evolving; however, the Holy Grail, which would be a breathable, moisture-resistant, long-lasting, and totally atraumatic formulation, has still yet to reach the market. The question is whether the dream adhesive will come from a subtle modification of an existing technology or through a real novel approach to adhesion to skin. As of today, two foreseeable paths for the evolution of skin pressure-sensitive adhesives look promising: biomimicry and the processing evolution of existing adhesive technologies to enhance product performances.

Scouring the natural world can be an effective way of finding design inspiration for new scientific innovation. When looking to design technology to overcome scientific challenges, chances are that evolution has beaten the researcher. It is not exactly a fair competition, as nature has been conducting research and development for billions of years longer than humanity. It is no surprise then that scientists and engineers across all disciplines are increasingly turning to biomimicry — looking to the natural world for inspiration. From jellyfish blooms to tropical rattan plants to marine worms, inspiration can be found almost everywhere. It is clear that when it comes to biomimicry, we are limited only by our own curiosity of the natural world.

The world of adhesion is no exception; an increasing number of ideas and products are being developed that have their roots in the mimicry of some natural process or biological adaptation. At a time when the costs associated with bringing a new product through the developmental pipeline to patients are skyrocketing, mimicking naturally selected, time-tested designs is an increasingly attractive strategy.

An acrylic adhesive. (Credit: Scapa Healthcare)

In terms of skin pressure-sensitive adhesives, an ideal design to replicate is the ability of geckos to climb vertical and overhanging walls. Geckos possess a complex set of specializations for adhering to smooth surfaces, which include stiff tendons that insert into their toepads. On their toepads, geckos possess millions of minute hairs, called setae, that act as a soft substrate that can conform to surfaces. In 2012, a research team unlocked the elegant and simple secret for adhesion and created a novel technology called Geckskin™. Similar technology could soon appear in the skin adhesive market. Surgical glues are another bio-inspired adhesive; there is a skin adhesive inspired by the Californian sandcastle worm. As its name suggests, this marine worm is famous for its ability to secrete an adhesive that it uses to glue particles of sand together, allowing it to build protective sandcastle-like tubes. This adhesive has been perfected through millions of years of evolution to set quickly and to work in wet environments without difficulty.

Printing and coating capabilities are rapidly evolving. One such development is 3D printing, which generally involves taking a digital model or blueprint created via software and printing it in successive layers of materials like glass, metal, plastic, and ceramic and then assembling one layer at a time. While 3D printing may seem a little unfathomable to some, especially when you apply biomedical engineering, many major manufacturers use it to create airplane parts or electrical appliances. Some of the most incredible uses for 3D printing are developing within the medical field.

Applications of this futuristic technology for medical purposes might sound like a Michael Crichton novel, but they are fast becoming reality. Bioprinting is based on bio-ink, which is made of living cell structures. When a particular digital model is input, specific living tissue is printed and built up layer by cell layer. If the biotechnology industry is working on printing living cells, then the appearance of 3D printed adhesives, including drugs and multiple functionalities, is also possible. This could redefine the whole adhesive environment, especially in regard to adhesives for disposable devices where shapes and multiple layers could be printed in one path.

Spray coating and pattern coating are other coating techniques that have the potential to advance adhesive development. While these are well known, they have not yet been fully utilized to coat in one path. By using spray or pattern coating, one could coat adhesives of various properties, thereby enhancing product performance.

Conclusion

Growth in the pressure-sensitive skin adhesive market is not stopping, and future iterations of wound care dressings, ostomy accessories, and wearable devices will push adhesive manufacturers to exploit and commercialize the novel ways recently investigated. Twenty years ago, skin adhesives were only acrylic or rubber-based; however, in recent years, great evolution has occurred. Adhesive advancements in the coming two to three decades might be even more mind blowing.

This article was written by Anne Wibaux, Senior Principal Scientist, Scapa Healthcare, Windsor, CT. For more information, visit here.