Wearable Tech and Prosthetics: Skin-Safe Platinum Silicone Mold Selection

Skin-safe is not a marketing grade. For wearables and prosthetics it is platinum chemistry, control of residuals, and a biological evaluation scoped to contact duration. A “skin-safe” icon on a craft RTV bottle is not the same file as a socket liner that will sit on residual limbs for ten hours a day.

This article is about selecting the rubber that becomes the skin-contact article (and the mold rubber only when the mold itself is the article, which it usually is not). The cleanroom Class 8 discussion lives in a different post. You can cast a skin-contact part in a clean, ordinary room if the risk file says so. You cannot skip the chemistry.

What “skin-safe” has to mean on a drawing

For a device or a prosthetic that contacts intact skin:

• Platinum (addition) silicone, not tin-cure. Tin catalyst and condensation byproducts are the wrong residuals for prolonged skin contact, and tin RTV is not the usual ISO 10993 path.
• No unapproved process aids on the part: no sacrificial mold release, no oil bloom used as a “finish,” no craft pigment from a hardware jar.
• Post-cure per the gum supplier’s healthcare schedule when volatiles matter. Wearable straps that still smell of siloxane are not finished.
• ISO 10993 scoped to the contact. Intact skin, limited (≤ 24 h), prolonged (> 24 h to 30 days), or long-term (> 30 days). Daily wear that a user repeats is cumulative. Cytotoxicity, sensitization, and irritation are the common surface-device starting endpoints. A chest-worn monitor for a week is not a Halloween appliance.

USP Class VI still appears on datasheets. Treat it as a historical screen. Do not use it as a substitute for a 10993 plan on a wearable you will sell as a device.

If the product is a costume appliance sold as novelty, the legal path may be a cosmetic or consumer framework rather than a medical device. That does not make tin-cure plus oil paint a good idea on broken skin. Say which regulatory hat you are wearing, then pick the rubber.

Wearable tech: two silicones in one product

Most wearables mix a skin-facing elastomer with electronics, adhesives, and textiles. The mold selection is about the elastomer part.

Watch straps, bands, ear tips, optical windows. Usually LSR, platinum, in steel. Hardness is a comfort versus buckle-hole tear trade. Too soft and the strap mushrooms and holds sweat; too hard and it bites. Optical HR windows want a clear, low-yellowing grade and a polish spec, not a general black 70A.

Patches and electrodes. Thin LSR or a gel. Skin adhesion is a separate adhesive chemistry. The silicone’s job is often a skin-conforming skirt. Residual inhibitor and unreacted oligomers are what show up in irritation complaints. Post-cure and a defined wash beat a “medical” label.

Encapsulated electronics on skin. The potting grade must be platinum, low-exotherm relative to the battery and skin, and evaluated for the contact. A thermally conductive filled silicone that is fine in a junction box can be the wrong extractables profile on a wrist. Do not reuse an industrial encapsulant file for a wearable without chemistry review.

Inhibition is a wearable-specific trap: some adhesives, silver inks, and foam tapes poison platinum at the bond line. Test the actual stack. A perfect LSR strap that will not cure against the chosen PSA is not a mold problem.

Prosthetics: drape, tear, and a liner that must last

Prosthetic work uses platinum RTV and platinum LSR in thicknesses and hardnesses that wearable straps never see.

External appliances and effects. Soft platinum (often the 00-20A neighborhood for flesh, firmer for structural cores) with a painted or intrinsic color. The mold is usually a platinum RTV skin of the sculpt. The *article* that touches skin should still be a healthcare-grade platinum, post-cured, pigmented with colors that belong in that file. Encapsulated foam cores and rigid urethane inner sockets are not the skin-contact material.

Liners, sleeves, and facial prostheses worn for hours. This is prolonged contact, sweat, and cleaning chemicals. Specify a grade with a healthcare statement, a cleaning protocol that the rubber survives (mild soap, not a solvent wipe that swells the liner), and a hardness that drapes without wrinkling into a pressure point. Tear strength is a clinical durability spec. A liner that rips at the distal end is a patient-safety problem, not a cosmetic one.

Anchors and overmolded frames. Bonding platinum to a rigid frame needs a primer that is in the biological file. A cyanoacrylate field-repair is not a manufacturing process.

Do not select prosthetic mold rubber by the same Shore 10A-versus-30A resin logic alone. The mold that copies the sculpt can be a high-tear 20A platinum RTV. The liner compound is a different SKU. Mixing those two purchases is a common procurement error.

Pigment, odor, and the shop habits that ruin a good grade

Skin-contact color must be a pigment or masterbatch listed for that healthcare gum, at a loading the supplier supports. Some reds and blacks inhibit platinum or bleed onto skin as a stain that looks like an irritation.

Powdered cosmetic pigments, artist oils, and “flesh paint” on the cured part take you out of the file unless you re-evaluate the finished, painted article. Intrinsic color in the mix is the cleaner path for devices. Extrinsic paint is common in film work; call it film work.

Talcum as a demold dust is a contamination and a respiratory habit. For skin articles, design the tool to release, then wash the part as specified.

What to write on the wearable or prosthetic RFQ

Contact: intact skin / mucosa / compromised skin. Duration: hours per day and days of use. Process: LSR or platinum RTV. Hardness range and why (drape vs structure). Healthcare gum family, post-cure, allowed pigments, ban on tin-cure and on sacrificial release against the part. Whether ISO 10993 (and which endpoints) is in the molder’s scope or the OEM’s. If the supplier answers with “platinum is skin-safe,” keep going until the duration and the pigment are on the same line.

FAQ

Is a craft bottle's "skin-safe" icon enough for a socket liner worn ten hours a day?

No. Skin-safe here is platinum chemistry, control of residuals, and ISO 10993 scoped to contact duration. A novelty appliance and a device are different regulatory hats; neither makes tin-cure plus oil paint a good idea on compromised skin. USP Class VI is a historical screen, not a 10993 substitute.

Why is tin-cure the wrong path for prolonged skin contact?

Tin catalyst and condensation byproducts are the wrong residuals, and tin RTV is not the usual ISO 10993 path. Ban unapproved process aids on the part: sacrificial release, oil bloom as a "finish," hardware-jar pigment. Post-cure when volatiles matter; a strap that still smells of siloxane is not finished.

How can a wearable be two silicones in one product?

The skin-facing elastomer is one spec; adhesives, textiles, and electronics are others. Optical HR windows want a clear low-yellowing grade, not a general black 70A. Industrial thermally conductive encapsulant files do not automatically transfer to a wrist. Test the actual stack for platinum inhibition against PSA, inks, and foams.

Why is liner tear strength a patient-safety spec, not a cosmetic one?

Liners and sleeves see prolonged contact, sweat, and cleaning chemicals. A rip at the distal end is a clinical failure. The mold rubber that copies the sculpt can be a high-tear platinum RTV; the liner compound is a different healthcare SKU. Do not buy them as one hardness choice.

Can I paint a wearable device with artist oils or flesh paint?

Extrinsic paint takes you out of the biological file unless you re-evaluate the finished painted article. Intrinsic color from a listed pigment at a supported loading is the cleaner device path. Encapsulated paint is common in film work; call it film work. Talcum as demold dust is also the wrong habit on skin articles.