Designing and Manufacturing Effective Wearable Stick-To-Skin Products
Introduction
Product ideas are a dime a dozen. Very few see the light of day and even fewer find commercial success.
Have you ever wondered why?
Finding a viable solution to an existing problem should be the driving force behind any product concept and its eventual design. But you’d be surprised how many products are pushed onto us every day without any consideration if they will bring a tangible improvement to the user’s life.
Ideally, the design thought process should have 6 stages i.e. empathy, defining, idealization, prototyping, testing, and implementation. A vast majority of “designers” fail to or prefer not to take this rigorous course.
The stick-to-skin industry has taken its due course over the years and admittedly, not every product has been a home run.
Bringing stick-to-skin products to the consumer is a major economic undertaking with far-reaching financial ramifications for the manufacturers. The way to make the endeavor profitable is to come up with products that can hold onto the skin for weeks at a time without causing any disruption to the user’s daily routine.
Our goal with this white paper is to share the core principles for ideating, designing, and manufacturing stick-to-skin products with the human-first approach – products that tackle problems and measurably enhance the user’s quality of life.
Finding the Right Balance: Comfort and Function in Stick-To-Skin Products
When it comes to designing stick-to-skin products such as patient monitors, the design process is quite different from that for conventional medical devices. You can easily identify some of these differences while others are rather subtle.
The challenge stems from the fact that the user must wear such devices for an extended time period as compared to usual stick-to-skin devices e.g. an adhesive wound dressing or ostomy flange.
Let’s take a closer look at the unique design requirements that must be considered to deliver the best of form and function.
Patient Autonomy
Wearable medical devices are often designed to be attached and activated in the patient’s home, which may be many miles from the nearest healthcare provider or institution. The patient must be able to easily apply the device to their body, ideally single-handedly.
Tack plays a key role here, especially for drug-delivery applications for which the patient needs a quick, secure attachment. The adhesives should be engineered for easy re-position ability i.e. the patient should be able to reapply the device on the correct spot.
Patient Mobility
While a hospitalized patient may be confined to a bed or somewhat limited in his or her movements, most patients using wearable devices will be going about their usual routines at home and work.
The wearable device should be able to stand up to the rigors of daily life such as exercising, showering, sleeping, and dressing.
An appropriate level of static shear and peel adhesion is necessary to ensure the device can stay secure during these activities. And unlike a wound dressing, it must not only stick to the patient but also have the strength to hold and secure the weight of the device itself.
While the device may weigh only a few grams, however, it can make a big difference in terms of the demands it puts on the adhesive.
Moreover, the skin will be sweating and regenerating cells, so device materials must be able to manage such bodily fluids while ensuring optimum device functionality.
Patient Discretion
In general, wearable medical devices should be small and unobtrusive. Again, within the in-patient setting, there are procedures and protocols that are very familiar (e.g., taping a catheter to a patient’s arm or attaching electrodes with wires to the chest). However, this approach becomes challenging in a non-medical setting.
For example, a patient may be willing to endure the inconvenience of having these monitors attached to their body for a surgical procedure and the immediate postoperative period or for a stress test that takes about an hour. But once back home in their professional and personal life, a device usually works best when it disappears under the clothing and can be all but forgotten. For wearable device design, this means the adhesive material must offer long-term wear, which requires excellent moisture management, peel adhesion, and static shear.
Seamless Device Connectivity & Data Transmission
A major reason why wearable device design is so complex is the diversity of expertise required to bring these solutions to market. Consider that many wearables bear more of a resemblance to consumer electronics than they do to medical devices.
Depending on the device’s functional footprint, it may need to be Bluetooth enabled or be able to dock with a USB port. Specialized software algorithms will be needed to convert the device’s digital data into meaningful clinical information.
A cloud computing infrastructure may be required to move and store enormous amounts of data. They are needed to deliver all of this data to healthcare providers and patients in a highly accessible, secure, and easy-to-digest way.
Adhesives: The Foundation of Wearable Stick-To-Skin Technology
Wearable devices have sensors that allow them to monitor patient activity and relay relevant information to the convened healthcare professionals.
However, the human skin presents a considerable challenge with respect to sensing and adhesion. Natural bodily oils, sweat, and hair may interfere with the device’s sensor and skin adherence.
These adhesives must act as a seamless interface between the device and the skin. It must not impact the device’s ability to monitor patient activity. Moreover, the patient should be able to wear the device for an extended time period without compromising the quality of life.
Adhesives and Wearability: How They Work Together in Stick-To-Skin Devices
Depending on the patient’s needs, they may choose to wear such a device for up to 3 weeks. This means the device must be able to withstand whatever the user’s daily life throws at it.
A reliable adhesive solution ensures that the device stays securely attached to the patient’s body for the intended period. Let’s take a closer look at how advanced medical adhesive solutions add to the utility of these devices:
Makes It Suitable For An Active Lifestyle
Just because a patient needs to be continuously monitored shouldn’t mean that they cannot commit to normal daily activities such as school runs, working out or a hike on the weekend. This, however, means that the device must deal with sweat, moisture, and an occasional bump. A reliable adhesive ensures that the device stays put and performs well under normal push and pull of the human body.
Ergonomics & Comfort
Let’s be honest. The idea of something continuously sticking to your body is not very appealing. That said, certain health conditions such as diabetes may require patients to wear glucose monitoring devices for weeks.A well-designed adhesive guarantees that the patient can wear the device comfortably for extended periods without the risk of skin rashes or irritation.
Adhesive Considerations For a Winning Wearable Device
While the patient benefits from the device itself, however, a reliable stick-to-skin adhesive is what enables the device to do its job. The following factors must be considered to ensure that the product meets the expected standards:
Moisture Management
Stick-to-skin devices are designed to be compatible with the user’s daily lifestyle. That means it must stand up to sweat, bodily fluids, and external moisture. To counter this, adhesive materials employ the following two methods:
- Fluid absorption – The adhesive takes on the moisture and holds it away from the skin to prevent irritation.
- Moisture-vapor transmission – Tiny holes in the adhesive and carrier material allow moisture to evaporate.
**Moisture management is a vast subject that merits a deeper discussion. We have devoted the next section to examine adhesive moisture management in greater detail.
Getting The Layers Right
Designing an effective stick-to-skin product is a complicated endeavor. Several layers of materials must come together before a product starts to take shape. The designers and manufacturers must get this delicate balance just right for the product to function correctly. For example, if an adhesive used in the skin contact layer is breathable but the outer layer does not allow moisture-vapor transmission, the product won’t perform as intended.
Wear Time
Depending on the patient’s needs, sick-to-skin products can be worn anywhere for a couple of days to several weeks. Forshort-term usage, the core adhesive compounds should be silicones, rubbers, hydrogels , and hydrocolloids.
And the backing should be foams, TPE, PE film, and PET. Stick-to-product intended forlong-term use should have adhesives made from solvent acrylics and UV-cured acrylics. And the backings should either be PU film or non-woven PET/ PU.
Skin Irritation
The slightest irritation can compel the wearer to abandon the device, thereby, defeating its entire purpose. Factors such as the wearer’s age, local climate, daily activity volume, device placement on the body, and product wear-time all equally impact the irritation level.Adhesive materials such as acrylic, rubber, silicone, and hydrogels offer the right balance for eliminating irritation. For the backing, the preferred material choices are PU/ PET/ PE film, on-wovens, PE Foams, and cloth.
Clarity Of Backing Material & PSA
Backing is the carrier to which the PSA layer is laminated. Moreover, it dictates the way the tape behaves and ensures that it stays in place. For example, when the tape is used to secure an IV tube in place.
Other applications include wound dressings, NPWT, cover tapes, and shower Tapes. Materials such as PU film, PET film, and PE films are used for an optically clear backing.
Material Availability & Strong Supplier Partnerships
Our partners include 3M, Avery Dennison, Berry Plastics, and Polymer Science. This ensures immediate material access and enables us to design and produce customized medical adhesive solutions far quicker and cheaper than most of our competitors.
Managing Moisture in Wearable Stick-To-Skin Device Adhesive Selection
Designers must choose the right adhesives to ensure a reliable wearable device. It can be helpful to have a basic understanding of the adhesive materials that serve as building blocks for wearables.
There are two overarching classifications that provide a helpful framework for the decision-making process.
- Adhesive Purpose: At a high level, there are three primary types of adhesive materials used in skin-worn devices.
- Skin-Contact Layer Adhesives: These are the materials that ultimately hold a wearable device directly to the patient’s body.
- Tie-Layer Adhesives: These materials tie, or connect, the different components of the wearable device together. While tie layers usually do not directly touch the patient’s skin, they must be compatible with the skin-contact layer. For example, if the skin-contact layer adhesive material is breathable, the construction-layer material must also be breathable.
- Cover Or Overlay Tapes: These are used as waterproof “covers” that are applied over the top of a stick-to-skin wearable device to protect the device from getting wet or dirty during everyday use. Typically, cover tapes are worn for 20 minutes up to 10 days before they are removed and a new cover or overlay patch is applied. They are a simple and inexpensive solution to protecting a wearable device.
- Adhesive Fluid Handling Method: The second big-picture factor to consider is how the adhesive material will manage bodily fluids such as sweat and oils. For wearables requiring extended wear times, moisture management is probably the single most important material performance characteristic. It affects both functionality and patient comfort – two core factors that dictate whether the device will be worn as intended and prescribed. There are two primary forms of moisture management:
- Moisture-Vapor Transmission: Tiny holes in the adhesive material allow moisture to move from the skin and out through the material to evaporate. Materials leveraging this approach are referred to as breathable.
- Fluid Absorption: The material absorbs moisture, holding it away from the skin so that it doesn’t cause irritation or maceration. The material contains ingredients that absorb the majority of the exudate (fluids), forming a gel within the material’s structure.’
The Importance Of Material Compatibility For Seamless Vapor Transmission
Throughout the wearable material selection process, it’s essential to evaluate the interplay between these core factors. Some skin-contact layer adhesives work well with some tie layer materials, and others are incompatible. Their compatibility often is directly related to their moisture management method.
For example, if a device maker wishes to use a breathable skin-contact adhesive, the manufacturer also should be sure to use a porous tie layer material or to include air channels in the design. Otherwise, fluids will be trapped and unable to evacuate and evaporate properly.
When vapor transmission is the preferred fluid handling approach, acrylic adhesive materials are a popular choice for the skin-contact layer. Acrylic adhesives can be coated onto thin foams or soft non-woven carrier materials. They are very stable, with few residual components that could leech into the skin over extended wear times.
For the tie layer, there are breathable transfer (or free film) tapes as well as some new double-coated tapes that provide reliable fixation for device components while complementing the breathability of the skin-contact layer.
How To Approach Adhesive Selection When Breathability is Not An Option
Some wearable device designs simply do not allow for moisture vapor transfer. Perhaps there is an airtight rigid plastic casing required to protect the device’s sensors and battery. Or in other situations, the target patient population may have extremely fragile or damaged skin, prompting the use of a gentle, silicone-based adhesive gel or an absorbent hydrocolloid.
In some cases, if a non-breathable device structure has to be used, a specialist converter can perforate certain materials to generate some breathability. When there is no means of ventilation, another solution is to position an absorbent hydrocolloid skin-contact material layer as an island beneath the sensor housing to capture moisture and keep tissue comfortable.
Other Factors to Consider for Adhesive Selection
Following are the factors that are equally important in ensuring reliable device adhesion:
- Static Shear:The ability to hold in position in the presence of shearing forces, such as bending and twisting movements. It is also known as cohesion.
- Peel:The ability to resist removal by peeling. This is also known as the level of peel adhesion.
- Tack: The ability to adhere quickly. For example, some pressure-sensitive adhesives may adhere almost instantly whereas others may need to be held in place with some light pressure for a short time to achieve optimal securement.
Some characteristics can be specifically engineered into the adhesive based on the specific formulation, coat-weights, and carrier materials used.
However, given the large material variety, the engineer must work closely with an adhesive materials manufacturer to determine the right adhesive for their particular application and end-user needs.