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Let's stick together: Why materials matter with skin-worn wearables

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Deepak Prakash, Thijs Janssens, Vancive Medical Technologies, and Paul Rosenstein, Pronat Medical, explain why material selection matters when it comes to skin-worn wearables.

Medical wearables are intrinsic to digital health’s exciting future, but before a new skin-worn device hits the market, it must pass muster for biocompatibility, patient comfort and performance. Material suppliers and converters can help device makers through the material selection process.

A digital healthcare revolution has started, and many across the medical ecosystem are engaged in it. This includes providers, patients, insurers, researchers, drug developers, materials suppliers, specialist converters, and, of course, medical device manufacturers. There also are many participants from outside the realm of the traditional medical establishment. These include cloud technology providers, software and app developers, battery and sensor specialists, mobile device makers and many others.

Wearable medical devices will play an important role in digital health’s evolution. They enable remote monitoring and anytime-anywhere care delivery, both associated with cost savings, convenience and, in some cases, better patient compliance with treatment plans. This ultimately can lead to more positive outcomes and higher-quality care. Skin-worn wearable devices offer a discreet way to gather vital signs or track physiologic metrics over extended time periods of one to two weeks, or even longer. These ‘smart patches’ also can be used for shorter tests and transdermal drug therapies.

As device makers race to bring new products to market, they can benefit from putting an early emphasis on material selection. Alliances with advanced materials suppliers and medical specialist converters can help this process move along smoothly. Ideally, device developers should forge partnerships with medical materials providers and converters well before they lock in specifications and file for regulatory approvals. That way, the partners can work together to evaluate the wearable project’s parameters and collaborate from the beginning on material selection. This approach reduces the likelihood that the device maker will need to switch out one material for another later in the game, which can cause significant delays.

First, a few basics

Navigating through wearable device material selection may seem like threading your way through a labyrinth of formulations, chemistries, constructions and regulations. To demystify the process a bit, it can be helpful to have a basic understanding of the adhesive materials that serve as wearables' building blocks. While there are many variables to consider in selecting the right adhesive materials, there are two overarching classifications that provide a helpful framework for the decision-making process.

1. Adhesive purpose. At a high level, there are two primary types of adhesive materials used in skin-worn devices — the kind that hold the device to the patient and the kind that hold elements of the device together. The former are called skin-contact layer adhesive materials. The latter are known as construction, or tie-layer, materials.

2. Adhesive fluid handling method: The second big-picture factor to consider is how the adhesive material will manage bodily fluids such as sweat. For wearables requiring extended wear times, moisture management is probably the single most important material performance characteristic. It affects both functionality and patient comfort, which ultimately drive whether the device will be worn as intended and prescribed. There are two primary forms of moisture management:

Throughout the wearable material selection process, it’s essential to evaluate the interplay between these core factors. Some skin-contact layer adhesives make excellent bedfellows for some construction-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 construction-layer material or to include air channels in the design. Otherwise, fluids will be trapped and unable to evacuate and evaporate properly.

When vapour 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.

Some wearable device designs simply do not allow for moisture vapour 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.

Biocompatibility across the value chain

On the material selection journey, device manufacturers need partners who can pave the way to wearables with unquestionable biocompatibility and safety. For example, a medical specialist converter will supply comprehensive documentation regarding how all device materials meet ISO 10993 standards. This is often demonstrated through biocompatibility reports documenting the material supplier’s test results for cytotoxicity, skin irritation and sensitisation according to these standards. Wearable device developers should expect nothing less.

Yet safety and optimal device performance ultimately depend on much more than biological evaluation of the material chemistries. The focus on biocompatibility and quality must extend well into the value chain. Suppliers’ facilities should be ISO 13485 certified, which means their operation’s quality management systems meet strict standards for the design and manufacture of medical devices. Top medical materials providers and specialist converters also will have extensive clean room and sterilisation capabilities. Only when all of these pieces come together, complete with end-to-end best practices and process controls, can the wearable device maker rest assured that the final product will be free from contamination and contain only safe components. 

Industrial design and a medical device mindset

The digital health market, and wearables as a high-growth category within it, have both benefited from multidisciplinary participation in an exciting wave of product development and commercialisation. Never before has the healthcare industry witnessed this level of involvement from entrepreneurs, scientists, venture capitalists and technologists from all walks of life. The resulting innovations blend diverse expertise, from consumer electronics to pharmaceuticals to social media. The best wearable product development teams leverage this eclectic mix of talent to think outside the box.

But for skin-worn wearables, it’s also important to be sure an industrial designer has a medical device mindset, including strong anatomical knowledge. When such an industrial designer is guiding development, and working closely with an experienced medical material supplier and specialist converter, a wearable device project can avoid some pitfalls and setbacks. As just a few examples, the industrial designer will anticipate and address concerns such as:

It is also important to note that up to 10% of the general population are likely to react badly to skin-worn adhesives, especially acrylic adhesives. This is a reality that needs to be borne in mind during clinical trials and wear tests.

In conclusion, skin-worn wearable devices will continue to support the rollout of revolutionary digital health initiatives. With patient care on the line, they must be comfortable, safe, easy to use and reliable. It’s important to devote attention to materials selection very early in the device development process. With collaborative partners from advanced materials and medical converting, wearable device makers can stay ahead of the curve in performance, biocompatibility and patient experience.

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