Adhesive polymers serve two primary purposes in wearable medical devices: Adhering the device to the patient (skin-contact materials) and holding together various device components (tie-layer, or construction-layer, materials). A brief overview of some considerations related to both material types follows.
|Within a wearable medical device, such as this skin-worn patch, adhesive polymer materials play a role in attaching the device to the patient as well as holding together components.|
A skin-contact adhesive material must adhere the wearable securely to the patient for the device’s prescribed-use period. Extended wear times are increasingly in demand for medical wearable devices: For example, some devices must be worn around-the-clock for seven to 14 days or longer.
Because the adhesive material may be in contact with the patient’s skin for a prolonged period, it must be biocompatible. Acrylic adhesives, when properly manufactured, have few residual components that could potentially leach out onto the skin and cause irritation.
Another major consideration related to extended wear time is moisture management. To ensure patient comfort and to protect the skin, it is essential to provide an escape route for sweat and other body fluids. There are two primary ways to manage moisture:
- Moisture vapor transmission (breathability)—moisture escapes either by diffusion or through tiny pores in the material and evaporates.
- Absorption—the material absorbs the moisture and holds it away from the skin.
The skin-contact layer material choice will depend on the device’s overall construction. Thin, conformable hydrocolloid materials can be a good option when the device design does not provide any outlet for vapor transmission. In some cases, the hydrocolloid layer can be positioned as an island at the center of the device, where it sequesters moisture.
When breathability is desired for moisture management, acrylic adhesive materials are a common choice. They are flexible in that they can be engineered to accommodate a wide range of wear times. Acrylic adhesives can be applied to a variety of carrier materials, including soft, fabric-like nonwovens made of polyethylene, polyester and polypropylene. Thin foams also are a popular carrier material because they conform well to the body.
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During device construction, it is necessary to adhere the skin-contact layer material to the device. This might involve bonding the material to a plastic sensor casing, for example. There are three main ways to establish this bond:
- Liquid adhesives (usually cured with UV light);
- ultrasonic welding;
- pressure-sensitive adhesive (PSA) tie-layer materials.
If using ultrasonic welding, it is important to ensure there is an appropriate match between the skin-contact and casing materials. The right combination of polymers is necessary for a secure bond.
If using PSA materials, it is desirable to use a double-sided adhesive material. Transfer tapes are one option. Because they do not rely on a carrier material, they are breathable and highly conformable. However, they also are difficult to process after their release liner is removed because they do not have any structural rigidity.
Double-coated tapes are a prevalent tie-layer option. Because they do have a carrier, they are well suited to various types of processing, such as die cutting and slitting. However, it’s important to carefully evaluate the carrier film. Nonporous films, for example, which eliminate breathability, would negate any moisture vapor-transmission benefits of the skin-contact layer. If a nonporous carrier material is used in the tie layer, the skin-contact layer would need to be a hydrocolloid or other absorbent material.
However, some double-coated tie-layer tapes are constructed with a porous carrier material, which would not hinder moisture vapor transmission. Therefore, these construction-layer materials would be a suitable counterpart for a breathable skin-contact material.
Adhesive polymer materials can serve very different purposes in wearable medical devices—it’s crucial to consider their properties holistically so that they can work well together to deliver the best results.
Neal Carty, PhD, is Director of R&D and Medical and Scientific Affairs for Vancive Medical Technologies. He can be reached at 440/534-3515 or [email protected].