The Constraint Is the Body, and It Does Not Negotiate

Every other product category lets you trade size for capability. A wearable does not. Past a certain thickness on the wrist it catches on sleeves; past a certain mass on the ear it falls out; past a certain rigidity on the chest it stops being worn after week two. The rejection is silent — the device does not fail, the user just stops putting it on — and it does not show up in a lab test.

So a wearable prototype program is built differently from a normal one. Instead of one prototype that gets refined, you build a series of cheap, deliberately incomplete models aimed at separate questions: what size can be tolerated, what shape stays put, what material the skin accepts, and what battery volume you can actually afford. Answering those in the wrong order wastes months.

Start With Volume Studies, Not Working Prototypes

Before any electronics exist, build solid blocks in the approximate size and mass of three or four candidate architectures. Printed shells filled to weight with lead shot or epoxy work fine. Give them to ten people to wear for a full day, including sleep if the product is meant for continuous wear, and collect the complaints.

This costs a few hundred dollars and typically eliminates half your concepts. Thickness is the usual killer: a wrist device over about 14 mm thick, or an ear device over about 9 grams, draws complaints regardless of how the mass is distributed. Distribution matters nearly as much as total mass — the same 40 grams close to the skin reads lighter than 40 grams standing off it.

Foam and machined block models are the cheapest way to run this stage, and the technique is described in foam models. Formalize the results into a hard volume budget before electronics design starts, because that budget is what the electronics engineer designs against.

Fit Iterations Have to Run on Real Bodies

Anthropometric data gets you the starting dimensions — 5th percentile female and 95th percentile male wrist circumference bound your adjustment range, and the reference tables are discussed in anthropometry in product design. But data will not tell you where a device rocks during a motion, or which curve makes a sensor lose contact.

Plan on four to eight fit rounds. Each round should be printed the same week it is designed, worn by at least twelve people spanning the size range, and evaluated over hours rather than minutes. Look for:

  • Pressure points that leave marks after two hours — these become sores after two days.
  • Migration: how far the device travels from where it was placed over a normal day.
  • Sensor contact stability during walking, typing, and arm swing, if you are measuring anything optical or electrical.
  • Trapped heat and moisture under the contact surface, which drives both discomfort and skin reaction.
  • Hair interference, which affects mechanisms and adhesive attachment in ways nobody anticipates on a CAD screen.

Ergonomic test models built specifically for this purpose — no electronics, just the contact geometry — are cheap and fast; the approach is in ergonomic test models.

Skin Contact Materials

Anything against skin for hours needs to be chosen for biocompatibility, not just feel. The standard framework is ISO 10993, and for a consumer wearable the relevant parts are cytotoxicity, irritation, and skin sensitization testing. A device marketed for medical use faces the full evaluation, described in ISO 10993 biocompatibility testing.

Material choices that hold up in practice:

  • Platinum-cure silicone — the default for straps and skin-contact surfaces. Chemically inert, tolerates sweat and sunscreen, and available in medical grades with existing test data.
  • TPU and TPSiV — better abrasion resistance than silicone and easier to injection mold, but some grades leach plasticizers and yellow with UV exposure.
  • Fluoroelastomer — premium sport strap material, excellent sweat and UV resistance, more expensive.
  • Titanium and 316L stainless for metal contact points. Avoid nickel-bearing alloys against skin; nickel sensitization affects a meaningful share of the population.

Beware the prototype trap here: printed flexible parts are not the same material as the molded strap you will ship, and they are porous, which changes both feel and skin response. Cast the strap in real silicone for any test running longer than an hour — the workflow is in casting soft silicone parts.

The Battery Volume Tradeoff

Battery is usually 30 to 50 percent of a wearable's internal volume, so every runtime decision is a size decision. Work it as an explicit budget rather than an afterthought.

Start from the wear model. A device charged nightly needs eighteen hours of margin. A weekly-charge device needs seven days plus reserve, which multiplies capacity by roughly eight and usually forces a different form factor. Users tolerate nightly charging on a wrist and hate it on a ring or a patch.

Then attack consumption rather than capacity. Cutting average current draw is almost always cheaper in volume terms than adding cells: duty-cycled sensing, aggressive sleep states, and batching radio transmissions routinely deliver three to five times the runtime at the same size. The firmware side of this is covered in low-power firmware and sleep modes.

Physically, curved and pouch cells buy volume back but need protection circuitry, support, and swell clearance of roughly 10 percent. The rules are in battery pack design for a product.

Straps, Clasps, and Cycle Testing

The attachment mechanism is where field failures concentrate. A user fastens a wrist device roughly twice a day, which is about 5,000 cycles over seven years, plus repeated flexing at the lug.

Test it deliberately and early. Build a simple fixture that opens and closes the clasp automatically, run it to 10,000 cycles, and inspect for wear on the detent, elongation of the strap holes, and fatigue at the strap-to-body junction. Add a static pull test of 60 to 100 newtons and a tumble test with the strap fastened. Cracking at the lug is the most common wearable warranty issue, and it comes from a stress concentration a fillet fixes if you find it before tooling.

Then test one-handed fastening while walking, with sunscreen and lotion on the hands, which is how the clasp actually gets used.

Getting a Wearable Built Right

Projects House develops wearables end to end — volume studies, fit iterations on real users, skin-contact material selection, battery and power architecture, and strap mechanism validation — through to a manufacturing package. Send your wearable concept and target wear pattern through our contact form.