Understanding the Basics of Adhesives
Selecting wearable medical device adhesives requires a solid understanding of the basics of adhesive. Sticking a medical device to skin is not as simple as it sounds. Numerous variables come into play such as the patient’s age, gender, race, diet, activity, skin condition, and climate. Other variables include the stretchability of the skin, contours of the body, perspiration levels and how much hair the patient has at the fixation site.
But wait, there’s even more. Wear time and load requirements will have an affect as well. This is the final installment of a three-part blog series covering the dos and don’ts of developing a skin-worn wearable medical device and the science of sticking to skin.
It is impossible to become an adhesive expert overnight. Thankfully, there are industry adhesive experts ready to provide material recommendations that will meet your product requirements. Directly contacting representatives of 3M or Avery Denison is an option. Another is to contact a contract medical device manufacturer that specializes in adhesive converting and manufacturing. They will have an adhesive expert to discuss your device and its requirements. It is good to get them involved early in the ideation phase. These experts know what is feasible and what is not. They will make recommendations based on your performance and Cost of Goods (COG) requirements.
In addition to the 3M and Avery Denison brands, there are many other options offered at lower prices. These vendors often have helpful samples in stock, and they will work with your design team to design and make prototypes. Once the design is finalized they will source the adhesives from their supplier then die cut, attach liner, assemble, and package according to your specification.
Prior to speaking with an adhesive manufacturer, it is helpful to know the commonly used terms and the basics of adhesives.
Pressure-sensitive adhesive (PSA) is used for adhering a device to the skin. It only requires light pressure to bond with a surface. PSAs come in different formats such as transfer tape, single-sided, and double-sided as seen in figure 1. Single-coated tapes have a PSA layer laminated to a carrier (also known as a backing) on one side and a removable liner on the other. The carrier can be a wide variety of materials such as foam, textile, woven, non-woven and more. Transfer tapes consist of a PSA layer and removable liner.
Double-coated tapes feature two layers of PSA laminated to either side of a carrier and protected on one side by a removable liner. Double-coated tape is commonly used for attaching a device on one side and to skin on the other. The adhesive on the side that attaches to the device can be more aggressive than the adhesive on the side that attaches to the skin.
Types of Adhesives:
There are four categories of adhesives commonly used today. Each has its own pros and cons and performs differently over time when worn on skin. A summary is presented in figure 2.
Another factor to consider is the level of adhesion. The adhesive must be strong enough to adhere well, but must release without causing trauma to the skin. Strength tests are done to measure the peel, tack, and shear strength. The results are found in the specification sheet of the medical tape. Adhesivetest.com has a definition for each component of testing. Here are a few definitions from the site:
Peel Adhesion – the force required to remove a label after a length of time, at a specific speed, and at a 180 or 90 degree angle. This measurement is usually measured in pounds/inch. (Figure 3)
Loop Tack – a looped label sample with the adhesive on the outside is lowered into contact with the test substrate, relying on facestock stiffness to regulate force applied. The label is removed and the force required to remove the label is measured in pounds/inch. (Figure 4)
Static Shear – A test sample is cut with a liner folded back on one end. The exposed adhesive is attached to a stainless steel panel and placed in a grip with the label facing down and a 250 or 500 gram weight suspended from the label. (Figure 3)
There are no magic numbers for adhesion specification that work for every use scenario. Variables such as the patient’s age, gender, activity, skin condition, and etc. will determine which adhesive is most appropriate. The specification data allows you to compare different adhesive options.
Transepidermal water loss:
If the device is to be worn for longer than a brief moment, moisture handling is another factor to consider. Transepidermal water loss is a natural occurrence of water that passes from inside a body through the epidermal layer (skin) to the surrounding atmosphere via diffusion and evaporation processes. The skin’s natural transepidermal water loss will cause excessive hydration of the skin underneath if the moisture cannot be transmitted. The moisture vapour transmission rate (MVTR) describes the breathability of the adhesive. In general the MVTR has to be greater than 500 g/m2 per day to be considered breathable. The level of breathability required depends on the user’s gender and the location of the fixation site. For example, women generally lose more water than men and certain body parts (such as hands and foreheads) have higher water loss.
There is a lot of information to consider when choosing the right adhesive for a stick-to-skin device. It is not a task to be taken lightly. Selecting the right adhesive is critical to the performance, wear time, comfort, and success of the device.
Christine Park is an Industrial Designer at StarFish Medical where she uses her knowledge of usability, colour, material, finish and emotion to design innovative medical devices for clients.