A wearable medical device spends days or weeks pressed against skin, moving with the body, collecting sweat underneath, and surviving showers. The material on the skin side decides whether the patient keeps wearing it. Devices are removed early for three reasons in practice. The adhesive irritates the skin, the housing feels rigid at the edges, or the assembly delaminates between the soft skin interface and the rigid electronics frame.
Silicone solves the comfort and durability side of that problem better than most materials. Choosing the right silicone form, hardness and bonding route is where programs succeed or stall.
What Wearables Demand From a Material
Skin contact wearable formats differ widely, and the material requirement follows the format.
Continuous glucose monitor patches, worn 10 to 14 days, with a sensor housing
ECG and EMG electrode pads, conductive or as electrode backing
Insulin pump and infusion set interfaces, in contact for 2 to 3 days
Neurostimulation and TENS electrodes, reused across sessions
Smart bandages and wound monitoring patches
Hearing aid domes, ear tips and earhooks
Sleep and activity trackers with long term skin contact
Four demands repeat across the list. The material must be soft enough to conform at the edges. It must stay stable through sweat, sebum and cleaning or reprocessing. It must bond permanently to rigid housings, flexible circuits or textiles. It must carry a skin contact biological package rather than a general purpose rating.
Specifying the Skin Contact Interface
Skin Contact Requirements and Test Standards
Skin contact drives the documentation before it drives the specification. A supplier that cannot supply the right biological reports will slow the program regardless of how good the molding is.
ISO 10993-5 for cytotoxicity
ISO 10993-10 or ISO 10993-23 for irritation and sensitization
ISO 10993-11 for systemic toxicity where the exposure route requires it
ISO 10993-18 for chemical characterization, which supports the toxicological risk assessment when extraction data replaces animal testing
USP Class VI as a recognized material screening baseline
REACH and RoHS statements for market access
ISO 11137 for devices that are gamma sterilized
Extended wear adds practical requirements that the standards do not measure directly. Adhesion retention over 10 to 14 days, moisture vapor transmission through the skin interface, and residue left behind at removal all influence clinical complaints. These are design tests rather than material certificates, and they should run on the finished wearable geometry.
Material Options for Skin Contact Surfaces
Silicone is one option in a broader material set. Each choice trades comfort against cost and process complexity.
| Material | Skin comfort | Adhesion to skin | Bonding to housing | Reusability | Cost |
|---|---|---|---|---|---|
| LSR, 20 to 50 Shore A | High | Not inherent, needs adhesive or gel layer | Excellent with primer or LSR-on-LSR | High | Moderate |
| Silicone gel pad | Very high | Moderate, low irritation | Requires a carrier or backing | Limited | High |
| Silicone pressure sensitive adhesive | High | High, peel controlled | Coated onto backing film | Low | Moderate |
| TPU film and foam | Moderate | High with acrylic adhesive | Good by heat or ultrasonic welding | Moderate | Low |
| Hydrogel | High when wet | High | Difficult, moisture sensitive | Very low | Low |
| Acrylic PSA | Moderate | Very high | Simple | Low | Very low |
The pattern that emerges in commercial wearable designs is a hybrid. A soft silicone layer or silicone gel pad sits on the skin side for comfort, a rigid or semi rigid housing holds the electronics, and an engineered bonding step joins the two. Acrylic adhesive remains cheaper and stronger on skin, and it also drives more irritation reports, which is why silicone gel and silicone adhesive appear more often in extended wear programs.
Hardness and Softness for Comfort
Hardness selection is a comfort decision with a manufacturing consequence. Softer grades conform better and demold with more difficulty.
| Application zone | Typical hardness | Reason |
|---|---|---|
| Skin contact dome or rim | 10 to 30 Shore A | Conforms to body contours, low pressure at edges |
| Sensor housing shell | 40 to 60 Shore A | Protects electronics, holds shape |
| Silicone gel interface | Below 5 Shore A or Shore 00 range | Maximum conformability |
| Ear tip and dome | 30 to 50 Shore A | Seal without pressure points |
| Overmold layer on rigid frame | 30 to 50 Shore A | Cushioning with moldable flow |
Below roughly 20 Shore A, LSR parts become delicate and mold release, ejection and packing all become harder to control. Where the design needs that softness, a silicone gel or a very soft LSR grade molded onto a stiffer substrate usually holds up better than a self supporting soft part.
Designing for Sweat, Movement and Wear Time
A wearable skin interface sees three continuous stresses. Moisture accumulates between the material and the skin. Shear loads from body movement pull at the edges. Repeated cleaning or reprocessing degrades surfaces.
Design responses that work in production:
Vent or channel the skin contact surface so moisture can migrate outward
Round the perimeter rather than leaving a sharp edge, since edge stiffness drives pressure marks
Choose a grade with low compression set if the part seals against the body, following ASTM D395 Method B or ISO 815
Test adhesion wet rather than dry, since peel values fall after sweat exposure
Specify the cleaning or disinfection cycle and test against it, instead of assuming the material will survive reprocessing
Silicone holds its properties from roughly -50 °C to 200 °C, which is far beyond consumer wearable exposure, so temperature is rarely the limiting factor. Moisture and mechanical fatigue decide service life.
Overmolding and Bonding to Electronics and Textiles
Bonding is where wearable silicone programs fail in the field. Four routes cover most production cases.
LSR overmolding onto a primed rigid substrate, the strongest and most common approach
Insert molding of flexible circuits and connectors inside the silicone body
Silicone adhesive bonding for joining cured silicone parts to housings
Mechanical retention with undercuts where chemical bonding is unavailable
Substrate choice matters. Polycarbonate, ABS and PBT bond reliably to LSR with the right primer system. Polypropylene and acetal resist bonding and need mechanical retention instead. Textile integration follows a similar logic, where a silicone coating or silicone ink anchor printed onto the fabric creates a physical lock for the molded element.
Where the wearable includes electronics, insulation and dielectric strength belong in the specification as well. Silicone covers that role naturally, and the grade selection has to satisfy both the skin contact and the electrical requirement at once. Material supply for soft skin contact parts sits within the liquid silicone rubber range, while thicker cushioning and profile elements come from solid silicone rubber.
Manufacturing Routes for Wearable Silicone
| Route | Best for | Notes |
|---|---|---|
| LSR injection molding | High volume sensor housings, domes, ear tips | Short cycles, tight tolerance, clean room friendly |
| LSR overmolding | Soft layer on rigid frame | Requires primer development and tool design |
| Compression molding HTV | Larger cushions, straps, low volume | Lower tooling cost, longer cycle |
| Silicone gel casting | Adhesive pads and electrode interfaces | Very soft, usually needs a carrier |
| Coating and lamination | Textile backed patches | High throughput on roll stock |
Volume is the deciding variable. A program at hundreds of thousands of units per year justifies LSR tooling with high cavitation. A clinical trial or a low volume specialty device often starts in compression molded HTV and moves to LSR at scale, and planning that transition early avoids a second round of expensive validation.
Validating the Skin Interface Before Tooling
Comfort and bonding problems surface late when they are tested late. A short pre tooling validation sequence costs little and prevents a mold rebuild.
Run a peel or adhesion trial on the actual substrate with the intended primer system, dry and after simulated sweat exposure
Mold flat plaques at the candidate hardness and check edge pressure behavior on a curved surface
Expose samples to the cleaning or disinfection cycle, then repeat the peel test
Confirm that the biological package covers the intended contact duration and body site
Walk the assembly sequence, since a bonded silicone part is difficult to rework without scrapping the electronics
Contact duration and body site set the biological scope. A three day infusion set and a fourteen day glucose sensor can land in the same skin contact category, and the longer wear period adds clinical evidence requirements rather than extra material certificates. Confirm both points with the notified body before committing to a grade, because a material change after design freeze reopens the biological evaluation.
Market entry route also shapes the file. Devices cleared under the FDA premarket process and devices assessed under the EU medical device regulation both expect a documented material history with lot traceability, and both accept ISO 10993 data generated on the final formulation rather than on an earlier development grade.
Where ANYSIL Fits
ANYSIL compounds and molds platinum cure LSR and HTV solid silicone in Guangzhou, with monthly capacity above 300 tons, 15 technical staff and pilot to production tooling capability. Wearable grades are supplied with ISO 10993 skin contact reports and USP Class VI data where required, and overmolding development supports housings, ear tips, cushioning layers and textile backed parts.
Send the wearable concept, substrate material, expected wear duration and annual volume. The technical team will propose a hardness and material form, run a bonding trial on your substrate, and return samples with the documentation list for your design file.
Start a wearable silicone project