Wearing it well: Challenges in medical wearables
As the medical field undergoes its transformative leap into a data-driven landscape, the ongoing emergence of medical wearable technologies pose interesting and sometimes formidable challenges to device designers as Stephanie Steichen, technical service and development specialist, DuPont explains.
The device attachment can oftentimes be overlooked as a critical parameter in overall device performance, function, and user experience. Although devices can be attached by clips, belts, or clothing, the comfort, ease, and flexibility provided by adhesive securement is typically preferable. The selection of an adhesive, therefore, is an additional and important criteria to consider early in the design of a wearable medical device.
Silicone adhesives have unique material properties and versatile chemistry that enables their use in medical device attachment. When compared to alternative adhesive offerings, silicones provide the necessary adhesion to securely affix dressings and devices with a non-sensitising, non-irritating, highly breathable, and skin-friendly formulation. There are two primary families of non-bonding silicone adhesives: soft skin adhesives (SSAs) and pressure sensitive adhesives (PSAs). The SSAs are lightly crosslinked silicone elastomers that form a soft, tacky gel, which has gentle adhesion to injured skin for scar/wound care applications. PSAs are a polycondensed polydimethylsiloxane/silicate resin network with higher adhesion levels recommended to firmly attach devices to healthy skin. Within each adhesive family there is a wide range of available adhesion levels and additional properties that can be tailored for the needs of a given application.
The selection of an appropriate adhesive requires the consideration and understanding of several critical parameters, which will, to varying degrees, affect the adhesive performance and, ultimately, the wear of the device. These parameters can be broadly classified into two categories: unmodifiable and modifiable. The unmodifiable parameters are the inherent properties of the skin, the user, and the intended application that will have an impact on wear, but cannot be manipulated in a significant way. Conversely, the modifiable parameters, such as adhesive material properties and dressing design, will also have an impact on wear but can be controlled and altered by the designer.
Unmodifiable parameters
Skin physiology – The interaction between the adhesive and the substrate upon which it is adhering is critical. In other applications, tight control and consistent preparation of the substrate is of vital importance to form the adhesive bond and, consequently, to final performance. The skin, however, is a dynamic and highly variable substrate, which provides a challenging surface upon which to adhere.
The composition of the skin can vary significantly between individuals and also between different locations on the body. Factors such as hair density, oil/sebum production, sweat glands/production, and moisture levels will modify the skin’s surface and affect the interaction with the adhesive. These surface irregularities may potentially disrupt the non-covalent bonding of the adhesive to the skin and negatively impact wear.
This variability can be further amplified by disease state. Numerous diseases affect the health, integrity, and overall condition of the skin. The presence of wounded skin can complicate wear, as it may modify the mechanical properties of the skin, introduce extraneous biologic fluids, and prevent aggressive solutions that could further injure the area.
User type – The age, activity level, and general health of a device user can have drastic effects upon the wear of an adhesive/device. A healthy and active user will inherently place more demands upon the adhesive than someone bed-ridden. These adhesives are not covalently bound to the surface of the skin and instead rely on a combination of intermolecular interactions to remain adhered. There is a limit, therefore, of how much force these bonds can withstand before adhesive failure occurs. Movement of the skin under the adhesive is unavoidable, but the amount and frequency will impact the wear duration.
Application/Use - In certain use cases, the location of the medical device cannot be adjusted. For example, an ostomy pouch must be repeatedly placed over the stoma, which stays in the same location. This prevents the designer, or user, from adjusting the device to a more comfortable position unperturbed by clothing and free of other surface irregularities or contaminants.
Although the above parameters provide a challenging set of conditions to work around, between the adhesive selection and device design, the designer has a host of modifiable parameters with which to adjust wear and device performance.
Modifiable parameters
Adhesive design
Chemistry and rheological properties – The chemistry and rheological properties of the adhesive control its propensity and subsequent ability to interact with the surface of the skin – a relatively non-polar and hydrophobic substrate. The chemistry dictates the types of intermolecular interactions, e.g. hydrogen bonding, van der Waals forces, etc. that can occur, while the rheological properties facilitate the intimate contact required for these interactions to occur. It is thought, for example, that PSA adhesion is driven by moieties containing hydrogen bonding acceptor and donor sites being placed in intimate contact with the contours of the skin by way of its flowable, thermoplastic nature. SSAs, although thermoset, allow for the same intimate contact due to their soft, gel-like structure.
Based upon the desired application, the base and cure chemistries can be carefully selected to obtain the distinct adhesive families, i.e. thermoplastic or thermoset, and to further refine the desired material properties, e.g. cohesion, adhesion, softness, etc., which can impact wear. Although complex, a robust adhesive design is an important consideration that can significantly improve overall device performance.
Permeability – The adhesive’s permeability to gas and moisture will affect both wear and user comfort. An occlusive dressing has the potential to cause irritation and could, if severe enough, have a deleterious effect on skin integrity and health. While silicones are highly permeable to gases, they are less permeable to liquid water due to their hydrophobicity. In regions of the body with higher moisture, such as the armpit, on active users, or in humid climates, the wear can be negatively impacted due to moisture accumulation under the dressing. The introduction of perforations in the silicone dressing can mitigate these effects and improve wear.
Device design – The device design includes the selection of the adhesive, but also its overall construction. An increase in adhesive surface area, an increase in adhesive thickness (for SSAs), and a decrease in device weight should all increase device wear. By maximising the ratio of adhesive surface area to device weight, one should theoretically improve wear. This is with one caveat, however, that the thicker the silicone adhesive, the lower the permeability and the more occlusive the attachment, which could negatively impact wear and user comfort. The occlusivity of the device component itself should also be a consideration.
The location of device attachment is also critical. Large surfaces with flatter contours, such as the chest and abdomen, that are less likely to be accidentally perturbed will provide the most desirable surfaces for wear. If surfaces with more substantial curves, such as the arms, are utilised, the device must be flexible enough to conform to the contours or be designed to otherwise accommodate the contours. This is to ensure the intimate contact required for good adhesion and wear.
The wear of a device on the skin is a complex interaction that is affected to varying degrees by numerous modifiable and unmodifiable parameters. A balance must be struck in every device design between these two classes to maximise wear, user comfort, and device performance.