Wearable medical devices — continuous glucose monitors, cardiac event recorders, transdermal drug delivery patches, ECG monitors, and skin-worn biosensors — must function reliably while adhering to or resting against human skin for periods ranging from a few hours to several weeks. The adhesive bonds inside these devices are not in direct skin contact, but the materials used in the housing construction are in proximity to the patient’s skin, and chemical entities that migrate from the housing materials through the device construction can reach the skin surface. For devices with extended skin contact (typically above 24 hours in ISO 10993 terminology), this migration pathway creates a biocompatibility requirement for every material in the device construction, including the epoxy adhesive used for internal bonding.
How Skin Contact Affects the Adhesive Specification
The ISO 10993-1 contact category for a wearable device housing in prolonged or permanent skin contact is “surface device — intact skin contact, prolonged or permanent.” For this category, the recommended biological evaluation endpoints include cytotoxicity, sensitization, and irritation as the standard battery.
For adhesive bonds inside the housing — not at the skin surface but within the device structure — the relevant question is: can chemical entities from the adhesive migrate to the skin contact surface at concentrations that could cause sensitization or irritation? The answer depends on the construction of the housing, the distance and materials between the adhesive bond and the skin surface, and the migration rate of adhesive extractables through intervening materials.
For a wearable device with a closed housing and a soft adhesive pad as the skin interface — the typical construction of a continuous glucose monitor or cardiac monitor — the housing materials are the relevant assessment. If the adhesive bond is inside a closed housing separated from the skin by a polymer housing wall, the path for chemical migration to the skin requires permeation through the housing wall material, which for most construction polymers (polycarbonate, ABS, polyurethane) at body temperature is extremely slow. The practical migration of epoxy extractables to the skin surface from inside a closed housing over a 14-day wear period is negligible for standard housing constructions.
For devices where the adhesive bond is at or near the skin interface — an adhesive bead at the perimeter of a skin-worn patch, a bond between the housing and a flexible skin-contact membrane — the migration path is much shorter and the contact assessment must be applied to the adhesive directly. In these configurations, the epoxy adhesive is functionally a skin-contact material.
Sensitization Risk in Skin-Worn Applications
Sensitization — the immune-mediated delayed hypersensitivity reaction that causes allergic contact dermatitis in sensitized individuals on subsequent contact — is the primary biocompatibility concern for prolonged-skin-contact adhesive materials. Epoxy resin monomers, particularly bisphenol-A diglycidyl ether (BADGE) and other glycidyl ethers, are among the most common causes of occupational contact dermatitis in industrial workers handling uncured epoxy, and are documented sensitizers in the dermatology literature.
The sensitization risk from a cured epoxy in a skin-contact medical device is much lower than from uncured resin, because curing reacts most of the glycidyl ether groups into the polymer network, leaving only residual unreacted groups. The residual BADGE content in fully cured medical-grade epoxy is typically very low — well below the concentrations shown to cause sensitization in animal models. But “fully cured” is the key qualifier: under-cured epoxy, or epoxy that was not allowed to complete post-cure before skin contact, has higher residual reactive groups and higher sensitization risk.
The medical-grade specification for skin-contact epoxy applications requires ISO 10993-10 sensitization testing on the cured material at the production cure schedule. This confirms that the actual residual reactive group content at the production cure conditions is below the sensitization threshold.
For sensitization and irritation test data for specific Incure formulations at defined cure schedules for skin-contact wearable applications, Email Us — Incure can provide test reports formatted for device regulatory submissions.
Skin Irritation: Chemical and Physical Mechanisms
Irritation in skin-contact medical devices occurs through two independent mechanisms that must be addressed separately.
Chemical irritation arises from extractable chemical entities from the device materials that contact the skin and cause a non-immune-mediated inflammatory response. ISO 10993-23 skin irritation testing evaluates the potential for direct chemical irritation using reconstructed human epidermis (RhE) model assays. Medical-grade epoxy with low extractable content and documented ISO 10993-23 performance provides evidence against chemical irritation for skin-contact wearable devices.
Physical irritation arises from the mechanical interaction between the device housing and the skin — pressure, shear, and occlusion. Rigid epoxy-bonded housing edges that press against skin during normal device movement can cause skin breakdown under the pressure point, independent of any chemical effect. This physical irritation mechanism is addressed through mechanical design — rounded housing edges, compliant interface materials between housing and skin, and appropriate housing weight and footprint — not through adhesive chemistry selection.
Occlusion — the restriction of moisture vapor transport from the skin surface by an impermeable covering — causes skin maceration under devices worn for extended periods. The device construction, particularly the seal between the device and skin, determines occlusion. The adhesive inside the housing does not contribute to occlusion; the skin adhesive and housing geometry determine vapor transmission.
Leachables in Sweat-Wetted Environments
Skin-worn devices operate in a sweat-wetted environment — the device underside contacts perspiration, and sweat enters any gaps or permeable interfaces in the device construction. For devices with adhesive bonds at the skin-contact interface, sweat provides an aqueous extraction medium that is in direct contact with the adhesive.
Sweat is slightly acidic (pH 4.5 to 7.5), contains salts, lactic acid, urea, and trace metals. Under prolonged contact, sweat extracts chemical entities from the adhesive more rapidly than dry contact. Extractables testing in artificial perspiration (ISO 105-E04 formulation) at 32°C to 37°C for the wear duration provides realistic data on what the skin is exposed to during extended wear.
For certification of skin-contact epoxy adhesive to ISO 10993 standards for wearable device applications, a complete evaluation including cytotoxicity, sensitization, irritation, and extractables in physiological media at body temperature provides the evidence needed to support extended wear duration claims in regulatory submissions.
Contact Our Team to discuss medical-grade epoxy selection, skin sensitization testing, sweat extractables data, and regulatory documentation for wearable medical device assembly applications.
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